Shared Channel Detail
Get details of a shared channel.
Resolves in this order:
1. ``SharedChannel.id``
2. ``SharedChannel.channel_id``
3. Lazy heal: a ``Channel`` whose ``parent_channel_id`` is shared. The
parent's share rows are linked onto the child sub-channel via
:py:meth:`SharedChannelModel.heal_for_channel_id`, and the resulting
SharedChannel is returned. Covers collab sub-channels created after
the parent chat was shared (e.g. an in-progress collab when share was
clicked, or one spawned later).
