Only allow access to the admin to admin users

A new class-based view, `AdminModelView`, is being added as a subclass
of `ModelView`. This new view adds an `is_accessible` method that will
block access to users who do not have the 'admin' role.
Add admin link back to site

This new link will allow users to return to the site without having to
manually change the URL.
Add link from site to admin

This new link will show up for any user with the 'admin' role. It will
allow them to easily access the admin without having to manually change
the URL.
Turn off site registration

Currently external users aren't supported, so there's no point in having
registration on. There are no links to it and the template would need to
be styled anyway.
Add change password page

Most of the work for this change is handled by Flask-Security, but a
custom template is being added to make it match the site better. Also,
rather than using Flask-Security's default behavior of sending a user to
the home page after they've changed their password, they will be
returned to their profile, which is where the 'Change password' link
lives.