Fix CPU spin on joining large room

checkIfAlone() filters the whole member list, which is fine until
we do it once for every membership event, then we have an n^2
problem. Move it into the rate limited function.

Fixes https://github.com/vector-im/riot-web/issues/7163

src/components/structures/RoomView.js

@@ -701,7 +701,6 @@ module.exports = React.createClass({
         }
 
         this._updateRoomMembers();
-        this._checkIfAlone(this.state.room);
     },
 
     onRoomMemberMembership: function(ev, member, oldMembership) {
@@ -717,6 +716,7 @@ module.exports = React.createClass({
         // refresh the conf call notification state
         this._updateConfCallNotification();
         this._updateDMState();
+        this._checkIfAlone(this.state.room);
     }, 500),
 
     _checkIfAlone: function(room) {