http: add "Use reverse proxy host" option

added "Use reverse-proxy reported "Host" when possible" option, which
will prefer using the "Host" or "X-Forwarded-Host" values instead of
"External jfa-go URL" in the web app. To do so, app.ExternalDomain/URI
are now functions which take *gin.Context (the latter optionally). The
protocol for the request is determined from X-Forwarded-Proto(col), so
make sure your proxy includes it.

The wiki will have been updated to mention the new option.

email.go

@@ -326,7 +326,7 @@ func (emailer *Emailer) confirmationValues(code, username, key string, app *appC
 		}
 	} else {
 		message := app.config.Section("messages").Key("message").String()
-		inviteLink := app.ExternalURI
+		inviteLink := app.ExternalURI(nil)
 		if code == "" { // Personal email change
 			inviteLink = fmt.Sprintf("%s/my/confirm/%s", inviteLink, url.PathEscape(key))
 		} else { // Invite email confirmation
@@ -394,7 +394,7 @@ func (emailer *Emailer) inviteValues(code string, invite Invite, app *appContext
 	expiry := invite.ValidTill
 	d, t, expiresIn := emailer.formatExpiry(expiry, false, app.datePattern, app.timePattern)
 	message := app.config.Section("messages").Key("message").String()
-	inviteLink := fmt.Sprintf("%s%s/%s", app.ExternalURI, PAGES.Form, code)
+	inviteLink := fmt.Sprintf("%s%s/%s", app.ExternalURI(nil), PAGES.Form, code)
 	template := map[string]interface{}{
 		"hello":              emailer.lang.InviteEmail.get("hello"),
 		"youHaveBeenInvited": emailer.lang.InviteEmail.get("youHaveBeenInvited"),