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.

testdata/webserver/domains.conf

@@ -0,0 +1,85 @@
+Domains:
+	mox.example:
+		LocalpartCaseSensitive: false
+Accounts:
+	mjl:
+		Domain: mox.example
+		Destinations:
+			mjl: nil
+WebDomainRedirects:
+	redir.mox.example: mox.example
+WebHandlers:
+	-
+		LogName: static
+		Domain: mox.example
+		PathRegexp: ^/static/
+		DontRedirectPlainHTTP: true
+		WebStatic:
+			# This is run from the http package.
+			Root: ../testdata/web
+			ListFiles: true
+			ResponseHeaders:
+				X-Test: mox
+	-
+		LogName: nolist
+		Domain: mox.example
+		PathRegexp: ^/nolist/
+		DontRedirectPlainHTTP: true
+		WebStatic:
+			StripPrefix: /nolist/
+			# This is run from the http package.
+			Root: ../testdata/web/static
+	-
+		LogName: httpsredir
+		Domain: mox.example
+		PathRegexp: ^/tls/
+		WebStatic:
+			# This is run from the http package.
+			Root: ../testdata/web/static
+	-
+		LogName: baseurlonly
+		Domain: mox.example
+		PathRegexp: ^/baseurl/
+		DontRedirectPlainHTTP: true
+		WebRedirect:
+			BaseURL: https://tls.mox.example?q=1#fragment
+	-
+		LogName: pathonly
+		Domain: mox.example
+		PathRegexp: ^/pathonly/
+		DontRedirectPlainHTTP: true
+		WebRedirect:
+			OrigPathRegexp: ^/pathonly/old/(.*)$
+			ReplacePath: /pathonly/new/$1
+			StatusCode: 307
+	-
+		LogName: baseurlpath
+		Domain: mox.example
+		PathRegexp: ^/baseurlpath/
+		DontRedirectPlainHTTP: true
+		WebRedirect:
+			BaseURL: //other.mox.example?q=1#fragment
+			OrigPathRegexp: ^/baseurlpath/old/(.*)$
+			ReplacePath: /baseurlpath/new/$1
+	# test code depends on these last two webhandlers being here.
+	-
+		LogName: strippath
+		Domain: mox.example
+		PathRegexp: ^/strip/
+		DontRedirectPlainHTTP: true
+		WebForward:
+			StripPath: true
+			# replaced while testing
+			URL: http://127.0.0.1:1/a
+			ResponseHeaders:
+				X-Test: mox
+	-
+		LogName: nostrippath
+		Domain: mox.example
+		PathRegexp: ^/nostrip/
+		DontRedirectPlainHTTP: true
+		WebForward:
+			# replaced while testing
+			URL: http://127.0.0.1:1/a
+			ResponseHeaders:
+				X-Test: mox

testdata/webserver/mox.conf

@@ -0,0 +1,13 @@
+DataDir: data
+User: 1000
+LogLevel: trace
+Hostname: mox.example
+Listeners:
+	local:
+		IPs:
+			- 0.0.0.0
+		WebserverHTTP:
+			Enabled: true
+Postmaster:
+	Account: mjl
+	Mailbox: postmaster

testdata/webserver/static/dir/hi.txt

@@ -0,0 +1 @@
+hi

testdata/webserver/static/index.html

@@ -0,0 +1 @@
+html