Fixed: overview page threw exception dump when not logged in.

Here's what happened: The Twig template for /overview would use the
`render()` method to fetch HTML fragments, and that call would fail
because the user wasn't logged in. However, the permission check in
/overview itself was broken, allowing anonymous users to view the
overview page, so instead of redirecting early in the page lifecycle,
Bolt would start rendering the overview template, run into the `render`
function, and that would throw an exception inside Twig, which would
then be dumped into the response.

I haven't fixed the actual exception dumping problem, just repaired the
permission check for /overview, so the problem just doesn't appear
anymore. It's still broken though.

Closes #1308