Add DEBUG=1 notice to server

[mariusandra]

Changes

Running PostHog with DEBUG=1 in production and behind a HTTPS proxy (e.g. nginx) causes issues with the toolbar. Under these conditions the Secure flag will be removed from the session cookie, causing browsers to either display a warning like so:

... or (soon?) to not acknowledge to cookie at all. This is especially the case with SameSite=None cookies that we use to get the session ID when running the toolbar on someone's site.

When DEBUG=1 is set, then settings.SESSION_COOKIE_SECURE is not set as we are assumed to run on HTTP. That is correct behavior, as turning on Secure when over HTTP would cause the session cookie to be ignored.

This PR adds a warning message that shows when PostHog is running with DEBUG=1. We should dissuade users from doing this in production. Hopefully those affected will act accordingly.

I'll make a separate PR for "running development over https" (probably via ngrok) later.

Checklist

• All querysets/queries filter by Team (if applicable)
• Backend tests (if applicable)
• Cypress E2E tests (if applicable)

[timgl]

I like this!