Bug 931242 fine grained permissions

Notable difference:

* EVERYTHING to do with LDAP is gone. Now anybody with a Persona account can sign in.
* All places where it was good enough to be signed in are now replaced with either checking that you're superuser or that you have a specific permission. 
* ALL admin URLs are only reachable by users with superuser status (1)
* I have defined there permissions to boot:
  * `view_pii`
  * `view_exploitability`
  * `view_rawdump`
* These are now effectively hardcoded and can't be changed unless with a code change
* I made a sample group called "Hackers" that have all of these permissions. 
* There's a whole Admin UI for adding, editing and deleting groups. 
* There's a new page (link in the lower right-hand corner) called ["Your Permissions"](http://cl.ly/SKSX) which lists all permissions that are available and which ones you have or don't have. 
* Now, on the login page if you get sent there it differentiates the text if you're signed in bug lack permissions. 
* Trying to load the Exploitability Report now redirects if you don't have sufficient permissions. 

(1) This means that once we launch, we need to run something like this:
```
$ ./manage.py shell
>>> from django.contrib.auth.models import User
>>> u = User.objects.get(email='kairo@mozilla.com')
>>> u.is_superuser = True
>>> u.save()
```