Refs #5911 Adds function elgg_get_subscriptions_for_container() and equivalent subscriptions service method.
Updated docblock for elgg_get_subscriptions_for_container()
Method getSubscriptionsForContainer() should take a container_guid instead of an entity_guid.
Added a unit test for getSubscriptionsForContainer().
Pass getSubscriptionsForContainer() a guid instead of an entity.
Added a test for getting subscriptions for a container when there are no notification methods registered.
Merge pull request #6112 from juho-jaakkola/container_subscriptions

Refs #5911 Adds function elgg_get_subscriptions_for_container()