Allow creation of components on container page

This commit implements STUD-1490, allowing creation of components
on the container page. It also enables the delete and duplicate
buttons now that new content can be created that would benefit.

Note that it also creates shared functionality for adding components,
and refactors the unit page to use it too.
Merge pull request #3579 from edx/andya/container-add-buttons

Allow creation of components on container page