Collections Django Admin + soft delete, hard delete, and restore

Description

Adds API methods for soft-deleting, hard-deleting and restoring Collections, and a Django Admin screen to allow superusers to hard delete any soft-deleted collections.

Also adds a convenience method get_collection_components, which makes it easier for ContentLibraries to interact with collection entities.

Supporting information

Testing instructions

Using the API, create a couple collections (python manage.py shell):

from openedx_learning.authoring import api
learning_package = api.create_learning_package("FAL-3809", "Learning package for FAL-3809")
api.create_collection(learning_package.id, "FAL-3809-1", title="Collection 1 for FAL-3809")
api.create_collection(learning_package.id, "FAL-3809-2", title="Collection 2 for FAL-3809")

View the collection in the Django Admin: (python manage.py runserver), and ensure you can:
- toggle the collections' "enabled" flag on and off from the list view.
- edit the collections' title and/or description.
- delete Collection 1 using Django Admin

Using the API, soft-delete, restore, and delete a collection (python manage.py shell):

from openedx_learning.authoring import api
collection = api.get_collection("FAL-3809", "FAL-3809-2")
assert collection.enabled
# Soft delete
collection = api.delete_collection("FAL-3809", "FAL-3809-2")
assert not collection.enabled
# Restore
collection = api.restore_collection("FAL-3809", "FAL-3809-2")
assert collection.enabled
# Hard delete
collection = api.delete_collection("FAL-3809", "FAL-3809-2", hard_delete=True)
assert not collection.id

Deadline

ASAP

Thanks for the pull request, @pomegranited!

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openedx / openedx-learning