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 the rest of the file does not adhere to this new policy, so there are three options:

* Apply the policy here, leave the rest of the file as is - there will be small region that adhere to new policy and the rest of the file that does not.
* Apply to entire file - too big diff (mostly whitespaces)
* Leave as is - the policy is not yet enforced.

I would argue that leaving it as is is a lesser evil among the three options. If you are convinced this policy should be enforced from now on - I'll convert the whole file than.

One more thing - it uses two spaces per indentation level, so I'll have to change this to four as well, resulting in virtually every line touched by whitespace modifications.