Group management

[gregoire-astruc]

Given the information provided by the SAML SP, the plugin should:

• Create the group(s) if they don't exist in phpBB.
• Add the user to the groups.

This rises two concerns:

• Who would be the moderator of the group?
• If a user changes group (being added to a new one or revoked from another), the plugin should reflect those changes.

However, it might not all be doable.