Fix permissions for the Pages section of the admin console. Closes #2060.

[tompollard]

As discussed in #2060, there is a bug the "Pages" permissions. Currently, it is not possible to assign users the correct permissions to view the Pages tabs in the console.

There are two issues:

1. The navbar HTML template is looking for user.change_staticpage and user.change_frontpagebutton permissions, which don't exist. These permissions belong to the physionet app, not users.
2. The view_redirects view is restricting access to a physionet.view_redirect permission, which doesn't exist. The redirect permission belongs to Django's redirects app.

Side note, but it isn't ideal that the navbar logic displays all three subitems {% if perms.physionet.change_staticpage or perms.physionet.change_frontpagebutton or perms.redirect.view_redirect %}, even if the user only has permission to view/edit one of the subitems. I think we can worry about this later.

It might be cleaner to display all tabs/pages admin users, even if the user doesn't have permission to edit/view the page. This way the user knows what functionality exists, and they can request access if needed.

[bemoody]

The view_redirects view is restricting access to a physionet.view_redirect permission, which doesn't exist. The redirect permission belongs to Django's redirects app.

It seems like our test suite should have caught this; probably it didn't because admin is a "superuser" and therefore is assumed to have all permissions, even those that don't exist. :/