[BandcampBridge] Add band and album feeds (#1317)

* [BandcampBridge] Add band and artist feeds

This can return a limited number of the most recent releases by a band,
or a single release/album. Each release may be given a unique article ID
depending on its track list with the "Releases, new one when track track
changes" option, which should make them show up as new articles when
tracks are added or removed. Releases may also be split up to individual
articles for each track with the "Individual tracks" option.

This uses and undocumented API from the Bandcamp Android app. It's much
faster than loading and parsing the website HTML, and seems to fail less
often with more relaxed rate limits. It's still far from perfect in that
regard though.

The "Individual tracks" option generates requests for each individual
track so that can quickly run into rate limits.

The "Individual tracks" option also has a quirk where tracks released
under e.g. a music label will have their artist set to the label instead
of the actual artist of the track. This is a limitation of the API.