Implement /user/@userid:domain?action=chat

This is a URL that can be used to start a chat with a user.
 - If the user is a guest, setMxId dialog will appear before anything and a defered action will cause `ChatCreateOrReuseDialog` to appear once they've logged in.
 - If the user is registered, they will not see the setMxId dialog.

fixes https://github.com/vector-im/riot-web/issues/4034

src/components/views/avatars/BaseAvatar.js

@@ -32,6 +32,7 @@ module.exports = React.createClass({
         urls: React.PropTypes.array, // [highest_priority, ... , lowest_priority]
         width: React.PropTypes.number,
         height: React.PropTypes.number,
+        // XXX resizeMethod not actually used.
         resizeMethod: React.PropTypes.string,
         defaultToInitialLetter: React.PropTypes.bool // true to add default url
     },