visibility filtering for groups and group profiles

[escattone]

mozilla/sumo#2718

This PR builds a bit on #7177:

• Adds a GroupProfile.filter_by_visible method that encapsulates the visibility filtering in a way that can be used for filtering Group queries as well as GroupProfile queries.
• Adjusts the PRIVATE visibility to include leaders as well as group members.
• Assumes that Group objects without an associated profile are always public.
• Adjusts the can_edit method to use get_ancestors in order to avoid the possibility of multiple get_parent queries.
• Adds a group-profile manager with the visible method for convenience.
• Uses the new methods within the groups views and the users profile view to restrict visibility to the groups involved.

Questions

• Do we want to add any visibility filtering in either of the following places? Probably not, but I'm not sure.
  

  kitsune/kitsune/messages/api.py

  Line 47 in 9e1ac11

  groups = Group.objects.filter(name__istartswith=pre, profile__isnull=False)[:10]

  
  

  kitsune/kitsune/wiki/forms.py

  Line 117 in 9e1ac11

  queryset=Group.objects.order_by("name").all(),