Add image field on element update APIs

See merge request !686

Ensure all elements exist before adding to the selection

See merge request !685

Ensure elements are unique in a selection

See merge request !684

Best class filter on element workflow

See merge request !673

Remove ElementType.hidden

See merge request !678

Allow removing an entity from metadata

See merge request !677

Add metatype docs

See merge request !680

Fix HTTP 405 for frontend views

See merge request !682

Fix recursive filter docs

See merge request !681

Deprecate ListRelatedElements

See merge request !679

Admin enhancements

See merge request !676

* Prevent admins from deleting a corpus using the admin,
  force them to use the deletion API instead
* Add type filter on metadata
* Remove revision from metadata list (100 less queries!)
* Add corpus filter on entity roles
* Show element type display name instead of ID in the elements admin

This will probably not prevent 500, 502 or 503 errors in prod, as that
is just caused by a lack of hardware resources to handle the amount of
data we have.
We could also have a policy of removing from the admin all the actions
that can be done using the API? That would "hide" a lot of issues.

Limit MLClass search terms count

See merge request !672

Bump Django 2.2.11

See merge request !675

Optional children count on element lists

See merge request !674

Limit ES query length

See merge request !671

Document CreateElement

See merge request !670

Remove the shell docker image

See merge request !669

Split selection tests to test_selection_api

See merge request !667