Video transcode should run on regular background task queue

[blissd]

Fotema has a background task runner (bootstrap.rs) which queues and starts the various tasks run at startup. Since face detection was added, this task runner has been refactored to allow non-startup tasks to be queued and run, too. However, the video transcode feature was written before this refactor so instead runs parallel to the regular background tasks. This means if a video transcode is started while background tasks are already running, then the transcode runs as the same time and the user sees two progress bars (one for normal background tasks, one for the transcode).

The video transcode background task should be integrated into the bootstrap.rs task runner which will prevent video transcodes running at the same time as over expensive background tasks and also remove the need for a second progress bar in the UI.