We shouldn’t import models into migrations because if the model changes
later down the line then the migration can’t be re-run at a later date
(for example to rebuild a database from scratch).
We don’t need to encode the content before storing it (we’ll always do
that before rendering/sending) so we don’t need to use
`BroadcastMessageTemplate`.
And given that no past broadcasts will have personalisation, we don’t
need to replace the personalisation in the template before rendering it.
So we can just copy the raw content from the templates table.
new broadcast messages will have content filled whether they have a
tempalte or not, but old ones won't so populate.
Stole the session constructor from 0044_jos_to_notification_hist.py
