Single curation mask layer

Expected behavior:
- user can create as many merging or excluding layers as they want before pressing 'save'
- on 'save', layers will be collapsed to one unified merging/excluding mask

Actual behavior:
- on 'save', only one of the layers is used for the merging/excluding mask

Might be good to discuss this w/ @thao-do before pursuing a solution

Update: we only want to have one merge and exclude layer active at a given time