Improve commission schema

[hyena]

The representation of commission classes and prices in the database still isn't great: The primary keys are weird, the indexing could probably be better, and it involves settings columns. I recommend improving it. Perhaps something like:

• Commission classes should get an actual primary key for itself so that classids can increment sanely.
• Commission prices should have a foreign key referencing that and a real primary key of its own.
• Commission prices should use an enum for its currency and a bool for addon.
• We need another table that lets artists associate their submissions with a commission class, so people can provide samples.

All of this is subject to what's discovered while digging into it. It will require mostly changes in the settings.py view_callables module, and in the commishinfo.py module and in the template.

If someone wants to be a hero, a stretch goal would be figuring out a commission queue representation, although I'm not convinced that'll be superior to just letting the artist maintain a markdown list.

[hyena]

Also the schema should permit artists to re-order their commission types. Making an UI to change it is not a requirement for this ticket, only making such an UI possible.