See #432: API for tags

See #432 (closed)

So, I expect this to be the last big merge request regarding tag support in the API. There will probably be a smaller MR specific to radios, an admin specific MR, and some minor tweaks here and there, but I consider the backend implementation mostly complete with this branch, from an end user perspective:

• Extend tagging support to albums and artists
• API to suggest tags on tracks, albums and artists (using our existing suggestion system)
• Expose tags in artist, album and track representations
• Enforce a (configurable) maximum number of tags per entity, to avoid performance issues or attacks

Time for user interface!