Allow destroying locally-stored submissions after

[jnm]

...POSTing JSON to an external service.

• WARNING: ADVANCED USERS ONLY. POTENTIAL FOR MAJOR DATA LOSS! *

Should you want to direct ALL submissions to an external endpoint WITHOUT
storing any user-generated data in KoBoCAT's own Postgres or Mongo databases,
set the following in your environment:

• ENABLE_DESTRUCTIVE_REST_SERVICE=True
• FAILED_REST_SERVICE_BLOCKS_SUBMISSION=True
• FORCE_REST_SERVICE_NAME=destructive_json
• FORCE_REST_SERVICE_URL=https://your-server/your-JSON-submission-endpoint

Closes #390

[jnm]

When updating to kobokitten, we should make ENABLE_DESTRUCTIVE_REST_SERVICE = True have the effect of completely replacing the available REST services list instead of appending to it; that way, there will be no risk of another REST service (e.g. KPI hook integration) trying and failing after the data has been destroyed.