[FEATURE REQUEST] options to disable Preview generation

[minoplhy]

Is your feature request related to a problem? Please describe.
From what I understand, Chibisafe will summon ffmpeg to generate thumbnails and previews if file size is less than 100 MB. but if hosted on small VPS, it can cause a very high load. In this example, ffmpeg processing around 25 MP4 files, most of which are less than 1MB

Describe the solution you'd like
It'll be great if there's an option to disable preview generation.

[lakema17]

25 MP4s being uploaded at the same time is crazy, but yeah I could get behind an option to disable preview generation for edge cases like this.

[Pitu]

Implemented in #647