316 deprecate multimask

[mradamcox]

Further work toward #316.

In-progress! Do not merge. Unresolved issues:

• Removing a mask from a Layer completely the MultiMask interface does not actually set that mask as None on the layer. This needs to be addressed still.
• Remove the frontend request to check if a layer is in the multimask (no longer needed)
• Before this is merged/depoyed, all multimasks should be pushed to layers again (manage.py oneoffs add-masks-to-layers), and that operation should be updated to remove any layer masks that are not present in multimasks