Fix bug 869480: Middleware for enforcing user bans.

This is a simple model for user bans, and a middleware which enforces
them. The middleware will log out any request from a banned user, and
show them a page which can explain that they're banned, and show the
reason for the ban, before letting them proceed to their requested
page.
Bug 809495: Ban users by username.

A simple view and form for creating a user ban. Is keyed to having the
add permission on the UserBan model.