This commit modifies the code paths the admin app uses to send one off
emails and text messages to also accept letters.
This mostly worked already, the two changes were:
- making sure that one-off letters are processed by the correct task,
from the correct queue
- one-off letters sent from a service in research mode don’t get put on
a queue and go straight to `delivered` (because we don’t want to send
them for real)
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