TNL-2619

Revert the removal of hint templates.
Move the 'tab' attribute in the new-problem yaml
files out one level, out of the 'metadata' section.

Extends the common capa response types (string, numeric, multiple
choice, checkbox, dropdown) with feedback and hint
capabilities. "Feedback" refers to feedback shown to the student when
they check the problem, looking at their specific answer. "Hints"
refers to a Hint button in LMS which the student can click at any time
to see hints for that problem. The implementation extends the markdown
syntax to include feedback and hints. There are new Feedback-and-Hint
specific templates in Studio when the author clicks to add a new
problem.

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.