Rewrite the Manage Extensions page.
The old Manage Extensions page broke with the static media

reorganization and Pipelining, as the enable/disable extension function

was wrapped in a closure and wasn't accessible.
An attempt to fix this spiraled out of control, and now there's a whole

new page. It looks exactly like the old page, but is backed by a set of

JavaScript objects that can call into the extension API to list, enable,

and disable extensions.
We'll be able to build on top of this later on for things like

interfacing with an extension store.
The only difference between the UI in this and the previous version is

that extension dependencies are no longer listed. We haven't even tested

the list since redoing the look of this page, and I'd rather rethink

later on how best to really represent this. As it is now, this shouldn't

really impact anybody.
Some of the CSS for this page was brought over from Review Board as

well, helping to keep the code all in one place.

Tested that loading the list worked.
Tested enabling and disabling extensions.
Tested Configure and Database links.
Enabled an extension, clicked Configure, and hit back. Saw the list reload.