Move chibis to costume model

[summertriangle-dev]

for #129

This moves all the chibi handling things from Card to Costume, including all forms and stuff.

Other details:

• Internally, chibi storage was changed to use a ForeignKey instead of ManyToManyField
• The live2d model is now nullable, costumes can have either a live2d model, chibis, or both, but not nothing
• Costumes without live2d won't show up on the main /costumes/ page
• The chibi view will open card ajax modals (if available) instead of the live2d viewer
• The migration has custom code so I've included it here
• When there's no live2d, /costume/id.../ pages and modals have no top content, we could do something fancy for the chibi images if you want