MBS-13832: Don't skip PDF files for frontiest check

[reosarevok]

Implement MBS-13832

Problem

Frontiest PDFs are not shown on the sidebar for releases/events (it just shows "we have no front, see all", even if we do have a front but it's a PDF). We originally skipped PDFs because they didn't use to support thumbnails (making them bad choices for the sidebar and the like). That changed many years ago though, and especially for events, it's not uncommon for the frontiest image to be a PDF.

Solution

This just removes the check in index_listing views that filters out PDF files.

Testing

None yet.

[mwiencek]

This looks good, but do we want to continue excluding them from the API's /front endpoint? I suppose there is a chance of breaking clients that don't expect PDFs from there, so it would be safer to preserve the existing behavior (and I'm not sure anybody has requested it be changed). If so, an artwork-redirect PR should be deployed first.

[reosarevok]

We don't have any documentation saying we do exclude them (https://musicbrainz.org/doc/Cover_Art_Archive/API / https://musicbrainz.org/doc/Event_Art_Archive/API). It makes sense IMO to have an option to skip (or include) pdfs, but I'm not sure how that would work - as far as I can tell artwork-redirect just uses is_front from index_listing when asked for the frontiest, and index_listing seems to set is_front with LIMIT 1 so it doesn't have an alternative non-PDF option for cases when a PDF is_front.

Would the suggestion be to implement such a thing on the MBS side, with more than one is_front option?