* Additional attributes for band_type, band_status, concert_count,
add_new_members, play_commitment, touring_option, paid_gigs,
hourly_rate, gig_minimum
* For joined table musician_instruments, remove the hard requirement
that they be joined to a user, rather a “player” that is polymorphic.
* For joined table performance_stamples, remove the hard requirement
that they be joined to a user, rather a “player” that is polymorphic.
* For joined table online_presences, remove the hard requirement that
they be joined to a user, rather a “player” that is polymorphic.
* Change models as appropriate with new attributes and modify
belongs_to / has_many directives as necessary.
* Fix existing usages of user_id to work with polymorphic player_id.
* Fix tests that use user_id
* Add new tests that exercise online_presence, performance_samples, and
instruments that target a band, rather than a user.
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