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.

mox-/config.go

@@ -198,11 +198,12 @@ func (c *Config) AccountDestination(addr string) (accDests AccountDestination, o
 	return
 }
 
-func (c *Config) WebHandlers() (l []config.WebHandler) {
+func (c *Config) WebServer() (r map[dns.Domain]dns.Domain, l []config.WebHandler) {
 	c.withDynamicLock(func() {
+		r = c.Dynamic.WebDNSDomainRedirects
 		l = c.Dynamic.WebHandlers
 	})
-	return l
+	return r, l
 }
 
 func (c *Config) allowACMEHosts() {
@@ -245,6 +246,9 @@ func (c *Config) allowACMEHosts() {
 		}
 
 		if l.WebserverHTTPS.Enabled {
+			for from := range c.Dynamic.WebDNSDomainRedirects {
+				hostnames[from] = struct{}{}
+			}
 			for _, wh := range c.Dynamic.WebHandlers {
 				hostnames[wh.DNSDomain] = struct{}{}
 			}
@@ -678,11 +682,13 @@ func prepareDynamicConfig(ctx context.Context, dynamicPath string, static config
 	}
 	checkMailboxNormf(static.Postmaster.Mailbox, "postmaster mailbox")
 
-	var haveSTSListener bool
+	var haveSTSListener, haveWebserverListener bool
 	for _, l := range static.Listeners {
 		if l.MTASTSHTTPS.Enabled {
 			haveSTSListener = true
-			break
+		}
+		if l.WebserverHTTP.Enabled || l.WebserverHTTPS.Enabled {
+			haveWebserverListener = true
 		}
 	}
 
@@ -991,6 +997,29 @@ func prepareDynamicConfig(ctx context.Context, dynamicPath string, static config
 	}
 
 	// Check webserver configs.
+	if (len(c.WebDomainRedirects) > 0 || len(c.WebHandlers) > 0) && !haveWebserverListener {
+		addErrorf("WebDomainRedirects or WebHandlers configured but no listener with WebserverHTTP or WebserverHTTPS enabled")
+	}
+
+	c.WebDNSDomainRedirects = map[dns.Domain]dns.Domain{}
+	for from, to := range c.WebDomainRedirects {
+		fromdom, err := dns.ParseDomain(from)
+		if err != nil {
+			addErrorf("parsing domain for redirect %s: %v", from, err)
+		}
+		todom, err := dns.ParseDomain(to)
+		if err != nil {
+			addErrorf("parsing domain for redirect %s: %v", to, err)
+		} else if fromdom == todom {
+			addErrorf("will not redirect domain %s to itself", todom)
+		}
+		var zerodom dns.Domain
+		if _, ok := c.WebDNSDomainRedirects[fromdom]; ok && fromdom != zerodom {
+			addErrorf("duplicate redirect domain %s", from)
+		}
+		c.WebDNSDomainRedirects[fromdom] = todom
+	}
+
 	for i := range c.WebHandlers {
 		wh := &c.WebHandlers[i]