create `/players/.../collections` endpoint

[skrungly]

some implementation notes:

• the collections.db file is really simple and should be stored as-is per user so that it can be retrieved as needed, possibly using /players/.../collections.db.
• create a new sql table for collections, which will assist with fetching all relevant map data in one query (and can include visibility preferences too).
• parsed collection data should be accessible at /players/.../collections.
• collection info should be mostly read-only (unless overwritten by a new import), but users should be able to specify which collections are shown on their profile, possibly by a put request.
• write tests first!