Add support for JavaScript extensions.
Extensions in Djblets can now define their JavaScript counterparts,

which, like the Python versions, can attach to hooks and augment

functionality. It is up to the consumer of Djblets to define hooks to

use.
This functionality requires Backbone.js and Underscore.js (or

compatible). These are not provided by Djblets.
The consumer must also call the init_js_extensions template tag at the

end of the body tag in all applicable templates, in order to initialize

extensions.

Tested this with some pending work for Review Board.