feature: disabling songs in the database

[icxes]

There should be a way to make some songs not show up on search/be requestable/be autoplayed while still remaining in the DB. The rationale for this instead of just deleting things is that it

• would let us disable the stuff that's currently on the DB but probably doesn't belong there little by little while still being able to revert changes if necessary
• would let some things stay in the DB that are fine when played by DJs, but less so when played by Hanyuu
• looking at the list of disabled stuff should actually give a pretty good idea of whether something is ok to accept or not when it's not perfectly clear to someone going through pending

[Wessie]

we can probably re-use the usable field in the database, but the current code maps this column to a bool and so can't represent another state. Will keep this as a post-1.0 issue for now