Unbind JCommune from using group IDs for pre-defined groups

At the moment our DB migration creates groups like Administrators with ID as a constant (13). And we use this ID in our code. We need:

1. Correct migration to insert groups only if they don't exist
2. Remove usage of IDs in the code and use group names instead

One of ways of testing that everything works:

1. Create a clean database
2. Start Poulpe and see that Administratos group exists in GROUPS table
3. Start JCommune and see that there is no new group with name Administrators