[DTOSS-10297] basic auth for non-production environments

[MatMoore]

Description

This PR adds a login page that authenticates using HTTP basic authentication, configured by environment variables.
This is integrated with django.contrib.auth so that request.user.is_authenticated is True. The user that is logged in is based on the username from the basic auth.

This page is only shown if BASIC_AUTH_ENABLED is set.

Login is now required for every view except the login page.

How to test locally

• run make seed again to load in the user
• set BASIC_AUTH_* environment variables (see .env.tpl for an example)
• when prompted, enter the username/password that match the env vars

Not done yet

• setting the environment variables in the development environment
• when we update to version 10 of the design system we will be able to show the logged in the user in the header
• when we get to implementing CIS2, I was thinking we will add a separate login page containing the button
• we talked about having the ability to switch users in non-production environments, which could be a separate page

Jira link

https://nhsd-jira.digital.nhs.uk/browse/DTOSS-10297

Review notes

1. This uses basic authentication as that's what we agreed on the ticket. But having done this, I feel like it would be simpler to just use form based authentication, as this way we can use the built in LoginView instead of the custom code.
2. Assuming we keep the basic auth, the way I've implement is to add a /test-login URL that triggers the basic auth. If the auth is successful it redirects to the homepage. But alternatively I could move this to a middleware that triggers on every request, and passes through to the view on success. This is slightly neater as it avoids a redirect, and would simplify our playwright tests. What do you think?
3. Can you think of a better way of naming the view, to avoid TestTestEnvironmentLogin in the tests?