Decide on which screen to show after login in one place

This follows from a small amount of refactoring done when RTS was introduced. Instead of setting the screen after sync, do it only after login.

This requires as-yet-to-be-PRd riot-web changes.

This includes:
 - initialScreenAfterLogin, which can be used to set the screen after login, and represents the screen that would be viewed if the window.location at the time of initialising Riot were routed.
 - guestCreds are now part of state, because otherwise they don't cause the login/registration views to update when set.
 - instead of worrying about races and using this._setPage, use a dispatch.

src/components/structures/login/Registration.js

@@ -192,7 +192,6 @@ module.exports = React.createClass({
                 const teamToken = data.team_token;
                 // Store for use /w welcome pages
                 window.localStorage.setItem('mx_team_token', teamToken);
-                this.props.onTeamMemberRegistered(teamToken);
 
                 this._rtsClient.getTeam(teamToken).then((team) => {
                     console.log(