Fix MergeService immediately triggering on PR creation

• Changes in 0.12.1 introduced a bug in which every time you create a new PR targeting a branch for which there wasn't a merge queue yet, the PR was immediately added to the new queue (upon queue creation) and scheduled for merging, even if that PR didn't have the integration label yet.
• This release also fixes the WallEView client app to now handle multiple queues.