Support querying by exact rating

[derat]

I'm not sure how I'd expose it in the UI, but it'd be nice if there were some way to query for songs by exact rating instead of just by minimum rating or unrated. Maximum rating might also be useful for finding potentially-misrated songs.

Behind the scenes, a minimum rating of 5 is already mapped to Rating = 5 and unrated is mapped to Rating = 0; it's just 1-4 that are inaccessible.

The server side of this is easy; I can just add a rating parameter to the /query endpoint and update the query package to add the appropriate equality filter.

If I want to add maxRating, that hopefully also wouldn't require a schema change:

• 0 is Rating = 0
• 1-3 are RatingAtLeast2 = false, RatingAtLeast3 = false, and RatingAtLeast4 = false
• 4 is Rating != 5

I'm not sure that I can use the != operator; https://cloud.google.com/datastore/docs/concepts/queries#not-equal says that it's only available in Firestore for Datastore mode (but I think I got migrated).

[derat]

!= doesn't seem to work, at least in dev_appserver, but I should be able to just use Rating < 5 instead.

[derat]

No immediate plans to expose this in the Android app, since I've only wanted it when looking for misrated songs so far.