Add support for defining, packaging and using static media bundles.
Static media in extensions has been pretty bad. It took work on the

developer's side to get static media installed and used, and there was

no consistency.
Now, the extension framework handles all this.
Extensions can define css_bundles and js_bundles attributes. These

follow the Django Pipeline bundle format, with all paths being relative

to the extension's static directory. The extension framework will

handle transforming these into paths that can be looked up using the new

static finders, which will find them correctly whether they're in a

'setup.py develop' environment, or from an installed package.
Bundle names won't conflict, as they'll also be transformed to be

namespaced by the extension ID.
If a bundle's name is "default", it will appear on all pages by default

(assuming thet template for the page loads extension bundles). This is

the replacement for the newly introduced js_files list.
The new ext_compressed_js and ext_compressed_css template tags allow

other bundles to be loaded. They're passed the extension and bundle

name. TemplateHook now passes the extension to the context of any

rendered templates, so these can be passed in.
This improves packaging as well. A project implementing the extension

support subclasses djblets.extensions.packaging.BuildStaticFiles and

defines a couple properties, and then a setup.py just needs to fetch the

cmdclasses provided by the project. Once done, any defined bundles in

any extension will be automatically run through Pipeline and placed into

the resulting package (or installed, if using setup.py install). This

makes it extremely simple to deal with static media in an extension.

Tested locally with an extension in develop mode and as an installed

package.
Tested with a devel server and a "production" server.
All tests were with a related Review Board change.
Unit tests pass.