Improved gallery cover lookup

[Ksrx01]

Small PR that changes how gallery covers are looked up. Which should solve (or alleviate) several open issues, including #3311, #1432, #1856, #2720 and #872

Right now only files ending exactly in cover.jpg are used as covers, any gallery missing such a file uses its first image.
These changes essentially replace CriterionModifierIncludes with CriterionModifierMatchesRegex (performance drop should be negligible), and introduce gallery_cover_regex in config.yml, which defaults to a value of (poster|cover|folder|board)(\.jpg|\.jpeg|\.png)$.

This makes the cover selection function by default much more flexible, but also allows it to be adapted.

Note: First time coding with Go, so I'm unsure if the way the configuration value is passed down to the function isn't total nonsense.

[DingDongSoLong4]

Looks good to me, the only addition I would make would be to also add an option on the settings page to avoid having to directly edit the config file. The docs should probably also be updated for these changes as well.

[Ksrx01]

I skipped the UI due to quite limited spare time and the need to familiarize myself a bit more with the project. Figured it couldn't hurt to already commit this as a rudimentary feature for power users. It is however my intention to implement this in a future commit, hopefully in the coming weeks.

[bnkai]

With latest commit looks and tests ok.

Couldn't notice any performance issues.
Gallery page loading times seem the same and the FindGalleries query doesnt seem to be impacted ( a few ms maybe but the timings arent consistent enough to compare )

[Ksrx01]

It's now possible to change the regex in the settings page. Contrary to what I said previously, I didn't remove the regex compilation check in GetGalleryCoverRegex() (config.go@652). Figure it's better this way, as performance isn't impacted in any meaningful way. Will remove it if deemed better to do so.

[DingDongSoLong4]

I did some testing and removing the regex check in GetGalleryCoverRegex() actually ends up being disastrous if an invalid regex is passed by directly editing the config file. The current version of stash has a bug (my fix in #3428) where an invalid regex causes all filters to be ignored - which means that the query for the gallery cover returns every single image in the entire database. For my database with >1M images, loading 40 covers on the galleries page literally maxed my CPU and took over 10 seconds.

I also measured the time that only the regexp.Compile call took and for me it was only around 50µs, so there is no real benefit to removing the check even after the bug is fixed.

This also inspired #3424, adding similar validation to all the other regex settings.