Dev: Modifications to make local dev easier

[jdu]

The following changes to the Reach airflow DAG in order to make local development / investigations / debugging easier.

1. [ ] If Reach DEBUG=1 env is set, save DAG tasks locally to a configured folder instead of S3 (this is to avoid local runs overwriting each other in S3 between developers)
2. [ ] Check for local EPMC dataset file and load EPMC from there.
3. [ ] Pull EPMC records for policy-mock to reduce load for EPMC task in the DAG and speed up testing.
4. [ ] Hide policy-mock in staging / production as the test server isn't running in staging/prod
5. [ ] Add more documents to the Acme test scrape target for easier local testing

[jdu]

This may be partially addressed by architectural changes proposed in #419