316 deprecate multimask field on LayerSets

[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 checkForExistingMask (no longer needed)
• Update mask syncing command (manage.py oneoffs add-masks-to-layers) to remove any layer masks that are no longer present in multimasks
• Before this is merged/depoyed, run mask syncing command to make sure everything is up-to-date