impr: detect development mode based on config instead of window.location

[fehmer]

Description

While working on my local dev I noticed the website is connecting to the real API when opening the local site using a non-local IP.

With this change we pick the backend url and the flag to detect development environment from the webpack config and make it available in constants/config

@Miodec please check if this is usefull and if it will work on production.

[fehmer]

The link to the backend configuration is still http://localhost:5005/configure. I can replace it with the proper backend url, but the configuration site seems to only work correctly on localhost.
If you want I can check the backend code to make the /configure url available when acessing over external IP. If this should remain as a security feature I'll leave it as is.

[monkeytypegeorge]

Continuous integration check(s) failed. Please review the failing check's logs and make the necessary changes. https://github.com/monkeytypegame/monkeytype/actions/runs/6431232587