[Feature] Manage users' rbac using user groups

[mawandm]

Description
User groups help manage user permissions in bulk.

Detail
Managing user permissions in bulk can be a challenge. The concept of user groups allows to add user permissions in bulk with simplicity. This feature introduces user groups.

Acceptance Criteria

1. A user group can be created.
2. A role can be assigned to a user group.
3. A user can be assigned to a user group.
4. A user assigned to the user group assumes all permissions from all roles assigned to the user group.