[FEATURE] New field "permission_group"

[dobromirdobrev]

Is your feature request related to a problem? Please describe.
We need a way to define if a group is a permission group. Permission groups will be used to administer events in the Illinois app. We also need to load / filter the groups by this new field.

Describe the solution you'd like
Add a new boolean field permission_group (false by default) which can be set / updated when creating / updating group. Also add this field to the filter when loading groups.

This may also need a bbs API for loading group admins by group id (for the Calendar BB).

Additional context

• Source: [FEATURE] Team management of sets of events  illinois-app#4696
• Calendar BB issue: https://github.com/rokwire/calendar-building-block/issues/528

Note:
Please, let's discuss it before we start implementing it.