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

Fix KeyError when ml_class is missing from a selection classification request

See merge request !666

Basic corpus classes search

See merge request !665

Remove hidden/folder type checks

See merge request !663

Do not use element's hidden filters anymore

See merge request !662

Bump GItPython 3.0.8

See merge request !664