fix: Each user should have one organization only

[OmarIthawi]

I noticed that we're investing a lot of time in supporting and testing for multiple-organizations per user.

This goes against the Tahoe 2.0 architecture of having one LMS organization per user.

Here's what we need to do:

• Helpers should expect only one organization per user.
• No multi-org helpers should be created.
• Multi-org tests should be removed.

@shadinaif I know that we're re-using existing models which limit our freedom in changing them, but is this model fix possible?

 class UserOrganizationMapping(models.Model):
-    user = models.ForeignKey(settings.AUTH_USER_MODEL, ...)
+    user = models.OneToOneField(settings.AUTH_USER_MODEL, ...)
     organization = models.ForeignKey(Organization, ...)

Required test changes:

The helper tests below adds two organizations for a single user, this shouldn't be done:

tahoe-sites/tahoe_sites/tests/test_helpers.py

Lines 26 to 47 in aa58ddc

	def _prepare_mapping_data(self):
	"""
	mapping:
	default_org --> default_user
	Org1 --> default_user -----> self.mapping points here
	Org1 --> user2
	Org2 --> user2
	Org3 --> None
	"""
	self.org1 = self._create_organization(name='Org1', short_name='O1')
	create_organization_mapping(user=self.default_user, organization=self.default_org)
	self.mapping = create_organization_mapping(user=self.default_user, organization=self.org1)
	self.org2 = self._create_organization(name='Org2', short_name='O2')
	self.user2 = UserFactory.create()
	create_organization_mapping(user=self.user2, organization=self.org1)
	create_organization_mapping(user=self.user2, organization=self.org2)
	self._create_organization(name='Org3', short_name='O3')
	# We have four organizations
	assert Organization.objects.count() == 4