Remove Duplicate Site Permissions

Fixes #186. Customizes the user admin to remove duplicate/multiple `SitePermission` records which could previously be created by double submitting the save form. To fix existing users they simply need to be saved again. I've created a Mezzanine issue to enforce this at the DB level and I'll be submitting a PR to fix it there in an upcoming release.

**Note:** `registration_salesforce/admin.py` was removed in 067554391826b903fbf746930908e65a27cbcf52 you need to ensure the .pyc file has been removed to see this change.