Handling of `Jugendleiter` group

[chrisflav]

Currently the name of the group of youth leaders is hardcoded to be Jugendleiter and needs to be manually created. Suggestions to improve this:

• Add a config variable youth_leader_group that defaults to Jugendleiter and that replaces all hardcoded mentions of Jugendleiter.
• Add this config variable to the various example settings.tomls.
• Document that this group needs to exist and that the name can be changed via youth_leader_group.
• Automatically create this group during the migration phase.
• Forbid deletion of this group (possibly with some way to manually circumvent it).