Award Emoji can be awarded on many things and it can be difficult to
determine which namespace the awardable belongs to.

Thus to centralize the logic that finds the custom emoji for an
awardable, we introduce a scope on the CustomEmoji model that accepts a
resource (being either a Project or a Group), determines it's root
namespace, and finds all CustomEmoji for it. In combination with the
previous commit, all the Awardable models respond to #resource_parent,
which can be provided to this new scope.

This change fixes award emoji on merge requests, epics, and issues in
nested groups.

Changelog: fixed