Use an in memory object queue for tape task.

Until we have proper journaling support and JSON error handling with
tape, use an in memory queue.
Disable offline trakt support for now.
Fix Manage List dialog hiding buttons with many lists.
Bump stable version to 5.1.
Latest translations from crowdin.
Increase activity stream thresholds to 90 days.

An intermediate solution until we can make it unlimited again (once it
performs faster).
Changes for 5.1 hotfix release.
Merge branch 'fix-5.1'