Update page privacy form with login/group options
Add unit tests for setting group-based permissions
Add a basic login page to wagtailcore
Implement front-end logic for group and login-based permissions
Allow overriding the frontend login template name with WAGTAIL_FRONTEND_LOGIN_TEMPLATE
Add documentation for customising the login page
Add message on login page for authenticated users who don't have access
Most of the samples were already 4-space indented, but a few were using 2-space,
which is both inconsistent and, when it happened with Python code samples,
incompatible with PEP8.
All `.. code::` instances have been changed to use `.. code-block::`,
and have been properly formatted. The syntax names have been normalised,
so all django templates use the `html+django` syntax, shell commands use
`sh`, and plain text uses `text`.
