Rework scenario duplication

[iripiri]

• Add usergroups via API (before: groups.yaml)
• Add users to usergroup via API
• Duplicate scenario(s) for user when adding to a usergroup (before: when using external authentication)

[iripiri]

Open TODOs:

• [x] remove reading in groups from groups.yaml (affected: start.go, config.go, user_test.go, authenticate_endpoint.go)
• [x] rewrite the test TestUserGroups in user_test.go
• [x] delete duplicated scenario when user is removed from usergroup (usergroup_endpoints.go)
• [x] make sure all duplicated scenarios are deleted when an entire usergroup is deleted
• [x] Update scenarios on mapping update ( e.g delete duplicates if scenario with duplicate = true is removed from mapping )
• [x] write tests for the functionialities in the two points above (usergroup_test.go)

@SystemsPurge @Anyandsi @al3xa23

[SystemsPurge]

@iripiri Is it ok to set up another branch and work there ? i don't think you would want things to be merged immediately

[SystemsPurge]

Should deleting a usergroup also remove access of users to the non-duplicate scenarios ? And if so, should we go all the way checking whether another usergroup is giving said users access to the same scenario ?