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.
No, blocks aren't required to have any javascript (and thus, aren't required to have an initialization function). Notice this same check above. I agree that the warning message should only be displayed if the block has some but not all of the initialization code, since that's really the only error condition.

I'm a bit unclear on why reordering depends on having an initialization function, though.