[Feature Request] Blacklist filenames for thumbnails

[nspitko]

Please describe your suggestion, and the problem it'd solve.
Thumbnails are super useful for a bunch of functionality in LRR, so it's generally important that LRR picks the right image. Unfortunately, some archives are... blessed... with a credits page, and LRR may accidentally chose that one instead of the first page or cover.

There's already a lot of logic trying to solve this, but it's a cat and mouse game as new groups show up and create their own cursed naming schemes. So instead, it might be easier to just let the user attack these en masse. usually it's a small number of groups doing this and they tend to use the same filename for all their releases, so the logic should be straightforward.

Add a text box that takes a comma separated list of filenames to ignore as cover candidates. We probably want to wildcard or ignore the extension, so something like coolgroupscans.*,overconfidentaitranslators.*. The wildcard might be overdesigning this, it could just be straight keywords, but I want to avoid a case where a group might use the cover name as part of the page name too, eg, coolscans.jpg, coolscans_p1.jpg, etc.

The ability to regenerate thumbnail on a specific archive would also be handy here for testing the rule before pulling the trigger on the whole DB, but this is more of a stretch goal.