Add option to disable cleaning database when files are missing

[blissd]

Fotema maintains a database of all the photos and videos it has discovered. Each time Fotema starts it will scan the file system for photos and videos, and if Fotema detects any database entries to files that don't currently exist, then Fotema will run a cleanup process for the missing files. Fotema will:

• Delete any generated thumbnails.
• Delete any detected faces (each face is saved to separate file).
• Delete any transcoded video.

I hadn't thought this to be a problem, but from several issues raised for Fotema I see a good number of users don't store their photos and videos locally like me, but store them on a NAS. I fear that for those users they will one day start Fotema while their NAS is unavailable and then Fotema will run the cleanup process for every photo and video in the users collection... and delete all the database data, thumbnails, faces, and transcoded videos that might have taken hours to generate.

I think Fotema should have an option to disable automatic cleaning of data when files are missing.

[blissd]

If you've been affected by this, then please let me know and I'll prioritize this work.