Publish sections/subsections/units from course outline. 

**Tickets:** [STUD-1911](https://openedx.atlassian.net/browse/STUD-1911), [STUD-1932](https://openedx.atlassian.net/browse/STUD-1932)

**Acceptance criteria:**
* Selecting the Publish option from the course outline should bring up a pop-up confirming the publication of all unpublished changes for the section/subsection/unit, with options to publish or cancel publishing. 
* All course content in that unit should be immediately be set to “Public” and be visible to students if the release date is in the past. 
* The course outline should update to reflect the published status after the bulk publish action.

Sandbox: http://tymofij.m.sandbox.edx.org/
Performance is not affected.
done, `_.extend` -> `.extend`