Add overrides on endpoints returning different serializers

Note that all of those changes only affect the API docs, not the API client or the API endpoints themselves.

• CreateDataImportFromFiles returns the created process instead of the request payload
• CreateExternalRepository returns the created process instead of the request payload
• CreateImportTranskribus returns the created process instead of the request payload (an existing override on create was ignored as CREATE is not an HTTP method)
• AddSelection now returns an element
  ⚠ It should return a list of elements, but adding many=True here causes Spectacular to think AddSelection has pagination and this cannot be easily fixed (upstream issue)
• CreateEntity now returns the created entity instead of the request payload
• Register now returns the created user instead of the request payload
• Login now returns the user instead of the credentials