Python API:
* Converts UsageKeyV2 object keys into component keys for use with the oel_collections api.
* Sends a CONTENT_OBJECT_TAGS_CHANGED for each component added/removed.

REST API:
* Calls the python API
* Receives a collection PK + a list of UsageKeys to add to the collection.

Addresses PR reviews.

* Collection contents -> Collection components
* Add warning about unstable REST APIs
* Wrap update_components view in convert_exceptions
* Raise api exceptions in api.update_collection_components
* Use usage_key in CONTENT_OBJECT_TAGS_CHANGED event

as oel_collections.get_learning_package_collections has been removed.

…-rest-api' into jill/collection-components-rest

so we can use the updated oel_collections.get_collections method.

…ction-components-rest.pre-rebase