Backend: REST API to publish all changes in a unit

[bradenmacdonald]

In content_libraries, implement a REST API to publish all changes in a unit. This must force-publish all components in the unit too.

If not done already in #1705 , ensure that both the "GET unit" endpoint and the search index can return info on whether or not the unit has unpublished changes and whether or not it contains unpublished changes (among its children). These can be combined into a single field as we don't need to distinguish those two cases on the frontend.