Fix the post-commit creation to also create a draft.

Using the post-commit UI would create a ReviewRequest object with public=False,
but wouldn't create an associated draft. This meant that the review request
looked like it was public but wasn't.

Testing done:
Created a post-commit review request and saw the "This review request is not
yet public" banner.

Reviewed at https://reviews.reviewboard.org/r/5780/
Improve the title for the Action Hooks page.

The Action Hooks page was called "ActionHook", but the page is actually
about a collection of action hooks. this is a trivial change to rename
it to "Action Hooks".
Fix displaying and publishing drafts for initial review requests.

A review request without a draft that is marked as public=False would be
shown with a draft banner on RB 1.7, but the work on 2.0 broke that.
This was partially due to a change when reorganizing the
review_header.html template, and was further hindered by some missing
logic in the views.

Now, a review request created through the New Review Request page
without a draft will once again have a banner, and the Publish and
Discard buttons will work again.

It also fixes an issue with the error callback for Publish. It turns out
that it didn't have the correct parameters.

Reviewed at https://reviews.reviewboard.org/r/5781/
Merge branch 'release-2.0.x'
Add release notes for 2.0 RC 3.

This covers all the fixes and improvements made in 2.0 RC 3. There's a
number of bug fixes, performance improvements, extensibility
improvements, and a couple experimental Web Hook additions in here.

Reviewed at https://reviews.reviewboard.org/r/5758/
Merge branch 'release-2.0.x'