Rewrite the My Account page.
This is an extensive rewrite of the page that provides better

organization and extensibility, based on the equivalent page in

RBCommons.
The new page provides a navbar on the side for navigating the various

account preference "pages," which are shown in the main content area.

Each "page" can show one or more forms for changing preferences.
The biggest change in content is the Groups page. Instead of a terrible

flat list of checkboxes, we now display every group in a clean list,

with buttons for dynamically joining or leaving the group. The group is

also filterable, making it easy to find a group you want.
The whole infrastructure is designed to be extensible. While the hooks

aren't there yet, everything below them are. Pages can be registered or

unregistered, and the forms on a page can also be registered or

unregistered. This will later let extensions provide their own

preferences for each user.

Tested all preferences.
Tested joining and leaving groups.
Tested filtering groups.
Tested custom pages and forms on existing pages.