Support parsing matrix.to links in the timeline with ?via= in them

This ends up being translated to ?server_name= in the matrix-js-sdk, although that has a bug at the time of writing. It converts `server_name: ['a', 'b']` to `?server_name=a,b` instead of `?server_name=a&server_name=b`

For reference: the `viaServers` option is routed through the 'join_room' action to RoomViewStore#_joinRoom which is passed directly to the js-sdk http-api#joinRoom function.

Next steps:
* Fix the js-sdk parsing
* Make the SDK generate matrix.to links with ?via=

src/components/structures/LoggedInView.js

@@ -64,6 +64,9 @@ const LoggedInView = React.createClass({
 
         teamToken: PropTypes.string,
 
+        // Used by the RoomView to handle joining rooms
+        viaServers: PropTypes.arrayOf(PropTypes.string),
+
         // and lots and lots of other stuff.
     },
 
@@ -389,6 +392,7 @@ const LoggedInView = React.createClass({
                         onRegistered={this.props.onRegistered}
                         thirdPartyInvite={this.props.thirdPartyInvite}
                         oobData={this.props.roomOobData}
+                        viaServers={this.props.viaServers}
                         eventPixelOffset={this.props.initialEventPixelOffset}
                         key={this.props.currentRoomId || 'roomview'}
                         disabled={this.props.middleDisabled}