Extract homeserver URL from fully qualified user id

Also lookup the .well-known entry and use it if available.

Change-Id: I609046f01860fd5e3ba8cb801006e6098a4ad840

src/i18n/de.js

@@ -4,8 +4,9 @@ export default {
   ...germanMessages,
   synapseadmin: {
     auth: {
-      homeserver: "Heimserver",
+      base_url: "Heimserver URL",
       welcome: "Willkommen bei Synapse-admin",
+      username_error: "Bitte vollständigen Nutzernamen angeben: '@user:domain'",
       protocol_error: "Die URL muss mit 'http://' oder 'https://' beginnen",
       url_error: "Keine gültige Matrix Server URL",
     },

src/i18n/en.js

@@ -4,8 +4,9 @@ export default {
   ...englishMessages,
   synapseadmin: {
     auth: {
-      homeserver: "Homeserver",
+      base_url: "Homeserver URL",
       welcome: "Welcome to Synapse-admin",
+      username_error: "Please enter fully qualified user ID: '@user:domain'",
       protocol_error: "URL has to start with 'http://' or 'https://'",
       url_error: "Not a valid Matrix server URL",
     },