Support resetting password via email

This adds a self-serve password reset page. Users can enter their username
and receive an email with a link that will let them create a new password.
If they've never set a password, it will send them a one-time login link
(building on #776) that will then take them to their Account Settings page.
If they don't have an email associated with their account, they'll be
instructed to contact the admin, so they can manually reset the password.

Includes changes to the stylesheet and database, so run:

make ui
writefreely db migrate

Closes T508