Feature/sckan-279 - Admin interface changes for region + layer

[D-GopalKrishna]

Jira ticket - https://metacell.atlassian.net/browse/SCKAN-279

AE -> AnatomicalEntity

Task description:

• Allow to add Synonyms from AnatomicalEntityMeta and AnatomicalEntityIntersections.
• Added a custom form to this. And allow a charfield with comma separated values as synonyms.
• This then triggers a post_save - django signal that saves Synonyms if it is passed along while saving.

• Add Synonyms to AnatomicalEntity in another tab. In AE - we can also create new simple entities (AEMeta) or RegionLayer Combinations, and link synonyms to them to create a AE.
• Can create a region layer combination (but they are not AE)
• Can create a region or a layer (But they are not AE - they are AEMeta)

Acceptance criteria (from the Jira Ticket):

• allow the admins to add a region through the admin interface - ✅ - already available.

• allow the admins to add a layer through the admin interface - ✅ - already available.

• allow the admins to add/edit a synonym for an anatomical entity (region, layer) - 🟠 - Done in this PR. Changes being - since Synonyms are connected to AnatomicalEntity - We cannot directly have synonyms for Simple AE/Region/Layer. Rather we have a custom form to get synonyms for AEIntersections and Simple Entities (AEMeta) and save them in AnatomicalEntity.

• allow the admins to add an intersection region+layer through the admin interface - ✅ - already available.

• allow the admins to pick for origins, vias, destinations either a simple anatomical entity or an intersection region+layer. - ✅ - already available - Users can pick the Anantomical Entities in origins, vias, destinations.

[D-GopalKrishna]

@zsinnema we do need this ->

def get_model_perms(self, request):
        return {}

to register the AnatomicalEntityMetaAdmin to be referenced by AnatomicalEntityAdmin.autocomplete_fields.

[D-GopalKrishna]

Brought back this here - since when saving AnatomicalEntities empty - it gave error with the a if () else b logic. Because indeed in that case it is not any of the two.

Additional FYI - I tried to add a constraint to not allow both to be empty. But still didn't work.