Add admin_groups to grant admin access to users of these groups

[Vendetta01]

This PR adds a new configuration option admin_groups, to grant admin access to all users that are members of these groups and thus lets you define admin access through ldap group membership. Its implementation is similar to allowed_groups.

This fixes #160.

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[firemdkfighter]

When this PR will be merged?

[Yayg]

:+1: for getting this merged! Thanks for the contribution

[mrdrelar]

plus for this PR. When it will be merged?