Add NextAuth to frontend

[AndyRae]

Changes

A significant update to Carrot - moving the Next app to be the actual frontend of the application, and no longer proxied by the Django backend.
The change is therefore that the "legacy" UI of Carrot will no longer work at all. This only used as a "fallback" UI, but this will be breaking change and form the basis of Carrot 3.0.
Related code that can be deleted will be done in follow up pull requests.

This change is particularly necessary for resilience, by uncoupling the frontend/backend.

Includes:

• Implements NextAuth to use Django credentials to login.
• Adds a simple login page on the Next app
• Removes all proxying from the backend
• Occasional fixes to make things work

Closes #870

Environment changes

Environment variables added:

Frontend

NEXTAUTH_URL=http://127.0.0.1:3000/
NEXTAUTH_SECRET=verycomplexsecretkey
NEXTAUTH_BACKEND_URL=http://127.0.0.1:8000/api/

Backend

SIGNING_KEY=secret

[github-actions]

Tests	Skipped	Failures	Errors	Time
25	1 💤	6 ❌	0 🔥	4.588s ⏱️