For the common case of not needing to reference the UserMessage row
later, and for being a stream without private history, the UserMessage
row is irrelevant. Convert `has_user_message` to a thunk, and defer
loading it unless necessary.
A user who was no longer subscribed to a private stream kept their
UserMessage row for a message sent while they were in it; this is
expected. However, they _also_ kept that row even if the message was
moved to a different private stream that they were also not subscribed
to. This violates the invariant that users without subscriptions
never have UserMessage rows.
This `if new_stream is not None` block was improperly indented,
causing it to only run if the propagation mode was not `change_one`.
Since the block controlled creation and deletion of UserMessage rows,
this led to messages being improperly still visible to members of the
old stream if they were being moved from public to private streams.
Clients also failed to receive `delete_message` events, so the
messages remained visible in their feeds until they reloaded the
application.
Alex Vandiver