Use background task priorities

Merged username-removed-26331 requested to merge grote/repomaker:94-task-priorities into master

The priorities of background tasks will be used to give the repo update (and publish) task a lower priority, so that all other tasks that still modify the repository get executed first.

This should also reduce the number of required updates and publish tasks, because before a new screenshot would be downloaded right after an update and then trigger a new update.

Now all tasks that cause updates are executed before the actual update happens, so only one update needs to be performed in the end.

This commit also lowers the priority of remote repo updates, so that all other tasks are done first.

Closes #94 (closed)