Reduce/Simplify the ENV variables used by Falcon

[n-riesco]

I've had a quick look at the ENV variables Falcon is using; mostly in backend/settings.js but also in:

• backend/main.development.js
• backend/headless.development.js
• backend/routes.js
• backend/menus.js
• app/store/configureStore.js
• webpack.config.*

We have some ENV variables that we don't seem to be using any longer:

• PLOTLY_CONNECTOR_HEADLESS
• PLOTLY_CONNECTOR_APP_DIRECTORY

And some default ENV values that break oauth for the headless app (because by default AUTH_ENABLED is true and USERS is []).

I have two proposals:

1. remove PLOTLY_CONNECTOR_HEADLESS and PLOTLY_CONNECTOR_APP_DIRECTORY
2. introduce PLOTLY_CONNECTOR_MODE and make USERS, AUTH_ENABLED, SSL_ENABLED, ... derived settings with defaults based on PLOTLY_CONNECTOR_MODE.

A list of possible values for PLOTLY_CONNECTOR_MODE:

• desktop (for running as an electron app)
• web (for running as a web app)
• onprem (for running OnPrem)
• test (for testing with sample-storage)
• mock (for testing with mock-storage)

@tarzzz @chriddyp What do you think of these proposals?

[chriddyp]

👍 sounds good to me!