Allow changing settings between localhost and production backend

[armenzg]

https://localhost:8011
vs
https://uplift.shipit.staging.mozilla-releng.net

[armenzg]

mozilla/release-services#786 needs to be fixed first.

I fixed something similar in mozilla-frontend-infra/firefox-health-dashboard@a4db7cd (Notice I'm using neutrino instead react-scripts) by adding something like this:

"start:local": "BACKEND=http://localhost:3000 neutrino start"
...
process.env.BACKEND || 'https://health.graphics'
...
await (await fetch(${SETTINGS.backend}/api/bz/regressions/missed)).json();

[armenzg]

Unimportant until we can actually set up the backend locally.
mozilla/release-services#786