Fix #1080: Admin users able to edit Superadmin

Adding a check so superadmin users are not shown on the list when the
user seeing it doesn't have superadmin privileges. Also, if a
non-superadmin user tries to edit a superadmin accessing it directly
from the URL, user gets kicked back to users list.
Updating code to Ushahidi standards
Updating permissions calls
Updating SQL query

Bit of a roundabout way to make sure Superadmin are excluded from admin
user administration for normal admins
Switch back to usering a subquery to eliminate superadmin users

In the end I think this is better because its more obvious what we're
doing. Its still not nice having the subquery, especially with table
prefixing in there. However I'm not happy repurposing access_level
for this.
Merge pull request #4 from rjmackay/1080

Switch back to usering a subquery to eliminate superadmin users
Merge pull request #1194 from ditorelo/1080

Fix #1080: Admin users able to edit Superadmin