Edit/Add Person -- Username to E-Mail Switch

Made changes/fixes to person.php -- the controller responsible for
allowing editing and addition of users.

Fixed a bug that was causing the "add" argument in the URL not to be
picked up, so the controller was forever stuck in edit mode and would
not allow new users to be created.

Switched over from usernames and e-mails to just e-mails.  Usernames are
no longer displayed, prompted for, or checked.

Added consistent checking for duplicate e-mails on postback, and added
graceful handling when a duplicate e-mail is detected -- an error
message is displayed and the user's data is retained in the form.
Schema Update

Removed the `username` field from the `users` table since it is
obsolete.
Receipt Links

Organization pages now have a link to receipt pages for each paid
payment.

Just as with all payment details, these links are visible only to admins
and superusers associated with that organization.

In the future -- when the ability to edit payments is added --
administrators' link will link to the payment edit page.
Sidebar

Added a sidebar with the navigation options which were previously on the
top navigation bar.

This sidebar is only displayed when a user is logged in, just as the top
navigation bar was (historically).