Studio Support for Content Libraries (SOL-1, SOL-2, & SOL-3)

@antoviaque @explorerleslie @cahrens

**Background**: This is the UI code to enable creation and editing of [Content Libraries](https://openedx.atlassian.net/wiki/display/SOL/Content+Libraries). Together with #6033, it implements SOL-1, SOL-2, and SOL-3. Once #6033 has been merged to master, this branch is to be merged to `content-libraries`, which will become the new feature branch for library support.

**Discussion**: Architecture discussed extensively on [the wiki](https://openedx.atlassian.net/wiki/display/SOL/Content+Libraries) and in meetings, then the revised proposal was presented to the [Arch Council](https://openedx.atlassian.net/wiki/display/AC/Architecture+Council) on Oct. 21 and given thumbs up.

**Partner information**: 3rd party-hosted open edX instance, for an edX solutions client.

**Merge deadline**: ASAP - several other PRs are ready and blocking on this.

**Dependencies**: Requires #6033.
Actually as Christina pointed out below, we don't need all of these for the library page, so what I'm going to do is just reduce the list of templates used for the library view. Let me know if you want something else. I agree moving to RequireJS Text or something would be nice in the future but is outside the scope of this PR.