Update the UnityGroup class to be more generic

[hakasapl]

In this system "PI" becomes a group type instead of a hardcoded group.

The UnityGroup class will need to be updated to be more generic to fit with the above model. In addition, that class will now need to check permissions for each operation, depending on who calls the operation to be made. This should also be part of the frontend, but the backend needs to verify in addition.

Currently, all pi groups are stored in ou=pi_groups - this will be renamed to ou=unity_groups, and will have all the old pi groups as well as the new group types in it.

Add a new column to the group_types table named prefix. For example, the PI group type's prefix is pi_. That prefix will be assigned at group creation time.

Rename the ou=pi_groups in phpldapadmin to ou=unity_groups to reflect the ldap change. For now you will not be able to add the group type and other custom attributes to ldap. This can be ignored and just mentioned as a comment for now.