Add endpoint to allow org team members to be removed

This is similar to the corresponding endpoint for services. However,
it is a little simpler since we don't need to worry about always having
at least one team member for an organisation.

The new dao function added, `dao_remove_user_from_organisation`, is also
simpler than `dao_remove_user_from_service` since we don't have any
organisation permissions to deal with.

app/dao/organisation_dao.py

@@ -137,3 +137,8 @@ def dao_add_user_to_organisation(organisation_id, user_id):
     user.organisations.append(organisation)
     db.session.add(organisation)
     return user
+
+
+@autocommit
+def dao_remove_user_from_organisation(organisation, user):
+    organisation.users.remove(user)