Add views and tests for the post-commit portion of the new review request UI.

This change adds a view to orchestrate creation of post-commit review requests.
It presents a commit browsing UI that allows users to choose a branch and see
the commit history from that branch tip, and then select a commit and create the
review request from it.

This is not yet consumed anywhere except js-tests, since the main new review
request UI change is coming later.

Testing done:
- Ran unit tests.
- Used this with my other pending changes.

Reviewed at https://reviews.reviewboard.org/r/4355/