This commit changes the ChatThreadsManager into a native
class instead of an ember service, and initializes it
for every ChatChannel model. This way each channel has its
own thread manager and cache that we can load/unload as
needed, and we also move activeThread to the channel since
it makes more sense to keep it there, not inside the chat service.
The pattern of calling setOwner with the passed in owner
from ChatChannel is adapted from the latest ember docs,
and is needed to avoid the error below when calling services
from the native class:
> Attempting to lookup an injected property on an object without a container, ensure that the object was instantiated via a container
It works well _only_ if we use our own getOwner wrapper
from addon/lib/get-owner, which is for backwards compat.
c.f. https://guides.emberjs.com/release/in-depth-topics/native-classes-in-depth/
