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.
Oh, I see from @e-kolpakov's email. The issue is that https://github.com/edx/edx-platform/blob/master/common/djangoapps/xmodule_modifiers.py#L95 is all conditional on the `js_init_fn`. I think that the right fix is to just only conditionalize `init`, `runtime-class` and `runtime-version`, and to always add the `block-type`,  `usage-id`, and `request-token`, even if a block doesn't declare any initialization.