Change where join_room_name UI entry is being cleared

This is my attempt at a bug fix for the issue #735 (closed).

The problem was that the UI entry "join_room_name" was being cleared before it was ever actually used in the function "join_to_room()" which was causing the malformed id since it was just an empty string.

My solution was to move the clearing of the entry to after that value is used in join_to_room() since this seemed like the simplest solution. Just note this would make it so that the UI is being updated from /fractal-gtk/src/ui/connect/join_room.rs to /fractal-gtk/src/appop/room.rs which might not be ideal but since the value was already being received directly from the UI element I thought this shouldn't be a problem.