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-/admin.go

@@ -401,6 +401,31 @@ func DomainRemove(ctx context.Context, domain dns.Domain) (rerr error) {
 	return nil
 }
 
+func WebserverConfigSet(ctx context.Context, domainRedirects map[string]string, webhandlers []config.WebHandler) (rerr error) {
+	log := xlog.WithContext(ctx)
+	defer func() {
+		if rerr != nil {
+			log.Errorx("saving webserver config", rerr)
+		}
+	}()
+
+	Conf.dynamicMutex.Lock()
+	defer Conf.dynamicMutex.Unlock()
+
+	// Compose new config without modifying existing data structures. If we fail, we
+	// leave no trace.
+	nc := Conf.Dynamic
+	nc.WebDomainRedirects = domainRedirects
+	nc.WebHandlers = webhandlers
+
+	if err := writeDynamic(ctx, log, nc); err != nil {
+		return fmt.Errorf("writing domains.conf: %v", err)
+	}
+
+	log.Info("webserver config saved")
+	return nil
+}
+
 // todo: find a way to automatically create the dns records as it would greatly simplify setting up email for a domain. we could also dynamically make changes, e.g. providing grace periods after disabling a dkim key, only automatically removing the dkim dns key after a few days. but this requires some kind of api and authentication to the dns server. there doesn't appear to be a single commonly used api for dns management. each of the numerous cloud providers have their own APIs and rather large SKDs to use them. we don't want to link all of them in.
 
 // DomainRecords returns text lines describing DNS records required for configuring