Add thumbnail dimensions to config

[Cohee1207]

Also groups config values for thumbnails in one area.

Checklist:

• I have read the Contributing guidelines.

[Wolfsblvt]

Works as described.
Settings migration also works nicely. Tested like two different scenarios, didn't care for more.

Tested different dimensions too and it works if you change the values and delete the thumbnails.

Question though. Is it intentional to not have a basic level of config value validation?
Both for the quality value that existed before, and the new dimensions.
If you set a valid number but not a valid dimension (like a negative number) it runs into an error on thumbnail generation and uses the original size image.
If you set one dimension to 0, it generates a png that is broken and can't even be opened by an image viewer.

I think having basic value checking and using the defaults as fallbacks would be easy enough to implement.