Add formal support for looking up static files for extensions.
This introduces support for checking an extension's static/ directory

for static media lookups. If provided, the contents will be checked when

looking up a static URL beginning with the extension's ID. This will

match regardless of the filesystem path for the extension, meaning it

will work nicely without copying files or symlinking during the

development of an extension.
There's a new static media finder, ExtensionFinder, that must be placed

in STATICFILES_FINDERS. This finder is responsible for looking up

files prefixed by an extension ID.
In a production setup, it's important to install media files into a

common place when enabling an extension, so that the web server can

serve them up without going through Django's static files registry.

For that, when enabling an extension, the static files will be copied

over, but only when settings.DEBUG is False. In a development setup,

/static/ is based on the finders and not the filesystem (so long as a

static/ directory does not exist), so it will just use ExtensionFinder

to look up, and not look through the static/ directory.

Tested with two extensions: One using the old htdocs-bundled media files,

and one using static media. Saw the warning on the htdocs one. In both

cases, media files were served up correctly.
Verified after enabling an extension using the static files that the

files were not copied over but were accessible through the /static/ directory

using the finders. Switched out of a DEBUG setup and saw that it installed

to the static/ext/ directory.