Supply named default queue for Celery.

Supplying this will prevent queue clashes between various ATAT sites
sharing the same Redis instance.

Note that the Celery documentation is currently wrong about the name for
configuring this:

https://docs.celeryproject.org/en/latest/userguide/configuration.html#std:setting-task_default_queue

It specifies `CELERY_TASK_DEFAULT_QUEUE`, but
`CELERY_DEFAULT_QUEUE` is the value that Celery currently looks for.
This appears to be fixed in on an upcoming release:

https://github.com/celery/celery/issues/5575

This is worth keeping an eye on, since the configuration key could
change in the future.

config/base.ini

@@ -5,6 +5,7 @@ CA_CHAIN = ssl/server-certs/ca-chain.pem
 CLASSIFIED = false
 CONTRACT_START_DATE = 2019-09-14
 CONTRACT_END_DATE = 2022-09-14
+CELERY_DEFAULT_QUEUE=celery
 DISABLE_CRL_CHECK = false
 CRL_FAIL_OPEN = false
 CRL_STORAGE_CONTAINER = crls