improve webserver, add domain redirects (aliases), add tests and admin page ui to manage the config

- make builtin http handlers serve on specific domains, such as for mta-sts, so
  e.g. /.well-known/mta-sts.txt isn't served on all domains.
- add logging of a few more fields in access logging.
- small tweaks/bug fixes in webserver request handling.
- add config option for redirecting entire domains to another (common enough).
- split httpserver metric into two: one for duration until writing header (i.e.
  performance of server), another for duration until full response is sent to
  client (i.e. performance as perceived by users).
- add admin ui, a new page for managing the configs. after making changes
  and hitting "save", the changes take effect immediately. the page itself
  doesn't look very well-designed (many input fields, makes it look messy). i
  have an idea to improve it (explained in admin.html as todo) by making the
  layout look just like the config file. not urgent though.

i've already changed my websites/webapps over.

the idea of adding a webserver is to take away a (the) reason for folks to want
to complicate their mox setup by running an other webserver on the same machine.
i think the current webserver implementation can already serve most common use
cases. with a few more tweaks (feedback needed!) we should be able to get to 95%
of the use cases. the reverse proxy can take care of the remaining 5%.
nevertheless, a next step is still to change the quickstart to make it easier
for folks to run with an existing webserver, with existing tls certs/keys.
that's how this relates to issue #5.

http/adminapi.json

@@ -649,6 +649,45 @@
 					]
 				}
 			]
+		},
+		{
+			"Name": "WebserverConfig",
+			"Docs": "WebserverConfig returns the current webserver config",
+			"Params": [],
+			"Returns": [
+				{
+					"Name": "conf",
+					"Typewords": [
+						"WebserverConfig"
+					]
+				}
+			]
+		},
+		{
+			"Name": "WebserverConfigSave",
+			"Docs": "WebserverConfigSave saves a new webserver config. If oldConf is not equal to\nthe current config, an error is returned.",
+			"Params": [
+				{
+					"Name": "oldConf",
+					"Typewords": [
+						"WebserverConfig"
+					]
+				},
+				{
+					"Name": "newConf",
+					"Typewords": [
+						"WebserverConfig"
+					]
+				}
+			],
+			"Returns": [
+				{
+					"Name": "savedConf",
+					"Typewords": [
+						"WebserverConfig"
+					]
+				}
+			]
 		}
 	],
 	"Sections": [],
@@ -2898,6 +2937,214 @@
 					]
 				}
 			]
+		},
+		{
+			"Name": "WebserverConfig",
+			"Docs": "WebserverConfig is the combination of WebDomainRedirects and WebHandlers\nfrom the domains.conf configuration file.",
+			"Fields": [
+				{
+					"Name": "WebDNSDomainRedirects",
+					"Docs": "From server to frontend.",
+					"Typewords": [
+						"[]",
+						"[]",
+						"Domain"
+					]
+				},
+				{
+					"Name": "WebDomainRedirects",
+					"Docs": "From frontend to server, it's not convenient to create dns.Domain in the frontend.",
+					"Typewords": [
+						"[]",
+						"[]",
+						"string"
+					]
+				},
+				{
+					"Name": "WebHandlers",
+					"Docs": "",
+					"Typewords": [
+						"[]",
+						"WebHandler"
+					]
+				}
+			]
+		},
+		{
+			"Name": "WebHandler",
+			"Docs": "",
+			"Fields": [
+				{
+					"Name": "LogName",
+					"Docs": "",
+					"Typewords": [
+						"string"
+					]
+				},
+				{
+					"Name": "Domain",
+					"Docs": "",
+					"Typewords": [
+						"string"
+					]
+				},
+				{
+					"Name": "PathRegexp",
+					"Docs": "",
+					"Typewords": [
+						"string"
+					]
+				},
+				{
+					"Name": "DontRedirectPlainHTTP",
+					"Docs": "",
+					"Typewords": [
+						"bool"
+					]
+				},
+				{
+					"Name": "WebStatic",
+					"Docs": "",
+					"Typewords": [
+						"nullable",
+						"WebStatic"
+					]
+				},
+				{
+					"Name": "WebRedirect",
+					"Docs": "",
+					"Typewords": [
+						"nullable",
+						"WebRedirect"
+					]
+				},
+				{
+					"Name": "WebForward",
+					"Docs": "",
+					"Typewords": [
+						"nullable",
+						"WebForward"
+					]
+				},
+				{
+					"Name": "Name",
+					"Docs": "Either LogName, or numeric index if LogName was empty. Used instead of LogName in logging/metrics.",
+					"Typewords": [
+						"string"
+					]
+				},
+				{
+					"Name": "DNSDomain",
+					"Docs": "",
+					"Typewords": [
+						"Domain"
+					]
+				}
+			]
+		},
+		{
+			"Name": "WebStatic",
+			"Docs": "",
+			"Fields": [
+				{
+					"Name": "StripPrefix",
+					"Docs": "",
+					"Typewords": [
+						"string"
+					]
+				},
+				{
+					"Name": "Root",
+					"Docs": "",
+					"Typewords": [
+						"string"
+					]
+				},
+				{
+					"Name": "ListFiles",
+					"Docs": "",
+					"Typewords": [
+						"bool"
+					]
+				},
+				{
+					"Name": "ContinueNotFound",
+					"Docs": "",
+					"Typewords": [
+						"bool"
+					]
+				},
+				{
+					"Name": "ResponseHeaders",
+					"Docs": "",
+					"Typewords": [
+						"{}",
+						"string"
+					]
+				}
+			]
+		},
+		{
+			"Name": "WebRedirect",
+			"Docs": "",
+			"Fields": [
+				{
+					"Name": "BaseURL",
+					"Docs": "",
+					"Typewords": [
+						"string"
+					]
+				},
+				{
+					"Name": "OrigPathRegexp",
+					"Docs": "",
+					"Typewords": [
+						"string"
+					]
+				},
+				{
+					"Name": "ReplacePath",
+					"Docs": "",
+					"Typewords": [
+						"string"
+					]
+				},
+				{
+					"Name": "StatusCode",
+					"Docs": "",
+					"Typewords": [
+						"int32"
+					]
+				}
+			]
+		},
+		{
+			"Name": "WebForward",
+			"Docs": "",
+			"Fields": [
+				{
+					"Name": "StripPath",
+					"Docs": "",
+					"Typewords": [
+						"bool"
+					]
+				},
+				{
+					"Name": "URL",
+					"Docs": "",
+					"Typewords": [
+						"string"
+					]
+				},
+				{
+					"Name": "ResponseHeaders",
+					"Docs": "",
+					"Typewords": [
+						"{}",
+						"string"
+					]
+				}
+			]
 		}
 	],
 	"Ints": [],