[Feature Request] Separate thumbnail storage

[ayasechan]

The thumbnail cache is very large compared to other user data.

Currently mango stores thumbnails in the same database as the rest of the user data. As a result, I have to backup the thumbnail cache at the same time as I backup the user data.

For me, all my comic files are local and the cost of generating thumbnails is so low that there is absolutely no need to backup the thumbnail cache.

I would like to be able to store the thumbnail caches in a separate path so that users can decide for themselves whether to back up those thumbnail caches or not.