This reduces the complexity of our dependency graph.
It also makes sub_store.get parallel to message_store.get.
For both you pass in the relevant id to get the
full validated object.
This change should make live-update code less brittle,
or at least less cumbersome.
Instead of having to re-compute calculated fields for
every change to a stream message, we now just compute
the fields right before we render stream settings UI.
This is mostly a pure code move.
In passing I remove an unneeded call to
update_calculated_fields in the dispatch code,
plus some tests that don't need them.
m-e-l-u-h-a-n
Steve Howell