Formalize husbandry group permissions

[k1o0]

Currently there is a hack for husbandry permissions in the BaseAdmin change permissions method:

alyx/alyx/alyx/base.py

Lines 382 to 388 in 95c2e40

	# [CR 2024-03-12]
	# HACK: following a request by Charu R from cortexlab, we authorize all users in the
	# special Husbandry group to edit litters.
	husbandry = 'husbandry' in ', '.join(_.name.lower() for _ in request.user.groups.all())
	if husbandry:
	if obj.__class__.__name__ in ('Litter', 'Subject', 'BreedingPair'):
	return True

This is currently undocumented and only weakly matches by group name. This may be fix issue #657 although this user would need to know how to create such a group in Alyx.

Suggested actions:

• Ascertain difference between stock manager and husbandry group (best to ask @charureddy)
• Move this set of lines to a mixin, or at least to the individual model admin has_change_permission methods
• Create the group in the set_user_permissions management command
• Document this group in the Alyx user guide (see issue Document Alyx user permissions system #883)