Refactor env vars for apps

[madwort]

Apps airlock & job-runner etc share one set of env files in ~opensafely/config. This is good for some shared config (ideally job server token, although I think that's not currently working as intended) - but this leads to accidents where airlock picks up env vars intended for job-runner e.g. opensafely-core/airlock#305 . Most relevant env vars are namespaced in airlock but this can trip up the unwary when adding new env vars.

[madwort]

As part of this, ideally I would like the values for secrets to be manually provided to backend-server somehow & then rendered into 02_secrets.env rather than manually maintaining the env file directly as currently.