Disable JavaScript extensions when running unit tests.

Extensions can interfere with the unit test suite, so make sure they're
not initialized on the js-tests page.

Note that a TemplateHook can still pose a problem, but that's a problem
for another day.
Expand upon some of the API resource descriptions.

The descriptions for some of our main API resources were pretty thin.
This expands upon them just a bit. Largely, it's in preparation for
further edits to these particular descriptions that are coming in
another change.
Document extension metadata field.

This simple change adds a paragraph on how and why one would want to add the
``metadata`` attribute to their extension class.

Testing done: Generated HTML docs.
Reviewed at https://reviews.reviewboard.org/r/4794/