Backend: Allow adding existing components when creating a unit via the REST API.

[bradenmacdonald]

Allow adding existing components when creating a unit via the REST API.

Probably the "update unit" API.

Requirements:

1. Components must be from the same library.
2. Allow re-ordering and removing components via the update API.
3. Enforce permissions.
4. Ensure the publish_status field in the search index indicates when the unit has or "contains" changes.
5. Ensure a component_count field in the search index is updated to show how many components the unit has.
6. Components in units are always unpinned - we don't need to implement "pinned components" in edx-platform, even though there's support for it in Learning Core.