Fix invalid og:image url when using S3 storage

One of great features of Funkwhale is the ability to easily share free music with anyone. However right now links to artist/album/tracks have just generic instance Open Graph metadata so when posted somewhere where links are expanded they all return Funkwhale logo and instance description. This looks confusing, especially if posted multiple times in a row.

It would be so cool if track/album/artist pages had their own OG metadata with image showing album cover (as artists currently don't have photos, maybe their first/last/random album?) and name displaying track/album/artist name. Description should probably return instance description as it does now.

This would beautify and simplify music sharing on social networks and messengers a lot!

P.S. For example, this is how BandCamp does it. There is even an embedded player similar to what Funkwhale already has, linked directly into OG and thus playable right from the preview.