Genre: update model and API

Subticket of #432 (closed)

What is the problem you are facing?

Update the model and API to have genre supported.

Several questions there :

• A genre field or tag ? Or both (main genre and sub-genres) ?
• Genre attributed to songs ? Albums ? Artists ? All of them ?

My suggestion is :

• tags : if I am right, already implemented. May be problematic to show albums / artists with lots of tags. We should find a way to have a main tag showed in short views (like "Blues" instead of "Blues", "Guitar", "Louisiana Blues")
• I would add tags to albums and artists only.

Interested by @eliotberriot thougts on this

What are the possible drawbacks or issues with the requested changes?

None.

Context

Allow to sort out music easily by genre.