Previously we kept all entities and their items (which are also
instances of _converse.DiscoEntity) in a flat array.
Instead, we should have a tree-like structure where items are stored
on the relevant entity (and recursively on other items).
This came out of the desire to let `converse-muc` use the API to
determine whether MUC is supported. However, we don't know the entity
JID before hand and I couldn't think of a good way to query all current
and future entities for a feature.
So `converse-muc` still does it's own thing without the API, but some
refactoring came as a result of attempting.