Improve the look and feel and extensibility of the login/register pages.
The login and register pages have looked pretty terrible for a while,

and weren't at all extensible. This change cleans them up quite a bit

(both the visual style and the underlying code) and adds reusable CSS

classes and support for template hooks.
It also no longer hard-codes the description telling you to use your

standard username/password if registration is disabled. Instead, any

auth backend can provide login instructions, regardless of whether

registration is allowed. The instructions will appear immediately below

the header for the page.

Tested both forms in Chrome and Firefox.
Tested both forms with global and per-field errors.
Tested with a simulated template hook adding new login options.