CRUD endpoints for EntityType

Refs https://redmine.teklia.com/issues/2687
Depends #1410 (closed)

As we will implement a management interface for the new EntityType, we need several new endpoints to create, update, delete and list their instances.

Please implement the following endpoints, along with their unit tests:

• CreateEntityType
  • name
  • color (optional), a random color is generated if no color is provided
• (Partial)UpdateEntityType
  • name
  • color
• DeleteEntityType
  • only when not linked to any entity, raise a 400 if any entity with this type exists
• ListCorpusEntityTypes
  • the serializer should list id, name and color for a given corpus ID

Only a corpus administrator can use the create, update and delete endpoints. A guest user can use the listing endpoint.