Remove sources from API endpoints

After the 0.14.2 release we can remove the source attributes from all the creation endpoints: we should only be able to create a new ML result with a worker version.

Concerned endpoints are:

• CreateElement
• CreateEntity (ner attribute)
• CreateTranscription
• CreateElementTranscriptions
• CreateClassification

In the end, the serializer field DataSourceSlugField should be removed.

added 1 deleted label

We should also handle CreateClassifications on this issue

added 8h of time spent at 2020-10-15

Closed with !1037 (merged)

closed