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.
@andy-armstrong I actually have no idea what that limit is for - the same combined limit was used for course creation, so I kept it in place for library creation. @cahrens set the limit with commit ff2d952d3963a0b77e2f02f8bbc51bd458ddad86 as part of STUD-726. I don't have permission to access that issue, but perhaps you or Christina can enlighten me.