Unable to login to on-prem from electron app/headless version #306

@n-riesco @chriddyp Stuck in a deadlock situation for On Prem login from Falcon.

To Clarify: This issue is not valid for the version we ship with On Prem. This is because we control the environment variables on On Prem.

Explanation:

• Open the electron app or web app. Try to login to an on-prem installation.
• It tries to check the username against PLOTLY_API_URL and fails because PLOTLY_API_URL is not set initially. https://github.com/plotly/falcon-sql-client/blob/0d774bc43600dcd219bf4f378c4a6c517d00f70e/backend/routes.js#L272
• Since it fails, the user is not logged in and hence PLOTLY_API_URL is not set because this request requires user to log-in to be able to save to settings.yaml.

Proposed solution:

• For electron app, save domain to settings.yaml as soon as user enters it and hits Log In. Since electron app does not require login to make requests, this will succeed.

• For the web app, PLOTLY_API_URL must be explicitly set as an environment variable.