This introduces the new Admin UI for Review Board. Major highlights include:

* A new look and feel.
* A new dashboard, showing the main models people generally need to work with (Users, review groups, default reviewers, repositories), some server information (Review Board version and cache stats), and Review Board news.
* The database models have been moved to a "Database" tab.

Once Extensions lands, there will be an Extensions tab for activating/deactivating/customizing extensions, alongside the "Dashboard" and "Database" tabs.

Further additions will be made in the future for easily changing server settings and possibly for allowing extensions to add dashboard elements.

Made sure modifying the database fields still works fine.

Tested the news feed links, and made sure news loading failed gracefully if the server was inaccessible.