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/Registration.js

@@ -26,6 +26,10 @@ import MatrixClientPeg from './MatrixClientPeg';
 import Modal from './Modal';
 import { _t } from './languageHandler';
 
+// Regex for what a "safe" or "Matrix-looking" localpart would be.
+// TODO: Update as needed for https://github.com/matrix-org/matrix-doc/issues/1514
+export const SAFE_LOCALPART_REGEX = /^[a-z0-9=_\-./]+$/g;
+
 /**
  * Starts either the ILAG or full registration flow, depending
  * on what the HS supports