Standardize errors about localpart structure

Fixes https://github.com/vector-im/riot-web/issues/5833

This also includes changing some Jira references that aren't searchable anymore, and a thing to replace the spinner on the SetMxidDialog as per https://github.com/vector-im/riot-web/issues/5833#issuecomment-445323177

src/components/views/room_settings/AliasSettings.js

@@ -130,7 +130,7 @@ module.exports = React.createClass({
     },
 
     isAliasValid: function(alias) {
-        // XXX: FIXME SPEC-1
+        // XXX: FIXME https://github.com/matrix-org/matrix-doc/issues/668
         return (alias.match(/^#([^\/:,]+?):(.+)$/) && encodeURI(alias) === alias);
     },