Improve our support for inline review UIs.

The original codebase for review UIs prepared the groundwork for
displaying a review UI inline into part of a page, without needing its
own full Review Board page. It was never used in practice, and its
design assumed it would still sit alone on its own page, but a simple
one embedded in an iframe.

The new design allows it to be embedded into any container on a page.
That makes it easier to include in popups, the diff viewer, etc.

The defaut template now places all scripts inside of a new
review_ui_scripts block, which is placed in the correct place in
base.html. base_inline.html is stripped down to just include the page
content and scripts.

The ReviewUI class now provides a render_to_string function, which
render_to_response wraps. This does all the rendering of the template,
based on the parameters and ReviewUI configuration.

There's a template tag to render the ReviewUI into a template. It's
unused in this change, but will be used in an upcoming change.
If there's only one repository, auto-select it for new Review Requests.

A lot of our users have only a single repository. In the new review request
page, if we're in this situation, auto-select the repository, to reduce one
click.

Testing done:
Set visible=False for all but one of my repositories. Went to /r/new and saw it
automatically select the remaining one. Un-hid my repositories and saw the
"Select a repository" message.

Reviewed at https://reviews.reviewboard.org/r/4472/