Add support for permissions specific to Local Sites.
Local Sites had no equivalent of our custom permissions, like

can_change_status or can_submit_as_another_user. It also didn't grant

these abilities to administrators of Local Sites.
Now, a LocalSiteProfile defines a permissions dictionary, which contains

all granted permissions for the user. Local Site administrators

automatically have all permissions for a Local Site.
To do this, we make use of an AuthBackend's ability to do custom

permission lookups. The main function for this, has_perm, can take a

parameter for the object permissions are being looked up on.
This parameter is unused by defaut and in fact not allowed on any

built-in backends. Django doesn't make any real use of it. The intent

appears to be to pass an object like a ReviewRequest to it, but there's

no real rules governing this.
For our usage, passing a LocalSite as the object will have it look up

that permission in the user's list of permissions for the LocalSite. All

callers must then do when checking permissions is to pass the LocalSite

(or None) when calling has_perm.
The 'perms' context variable for templates has also been replaced with a

version that will pass the LocalSite for these requests.

Unit tests pass.
Manually tested closing a review request and editing one as a user with

those permissions, and as an administrator.