Limit the valid list of LocalSite admin permissions.

LocalSite admins were being shown as having all available permissions,
meaning that some code would appear to grant them permission (typically
for display) while others would disallow (when actually factoring in the
permission for anything real).

The main case where this was apparent was that LocalSite admins were
seeing "Delete Permanently" for review requests, though the option would
not work.

We now have a pre-defined list of valid permissions that LocalSite
admins can have. This was collected from all the call sites in the
codebase where permissions are checked. No other permissions are
considered available to the admin.

Reviewed at https://reviews.reviewboard.org/dashboard/?view=outgoing