Use environment variable to determine whether to filter dead links by default; fix flaky tests

[sarayourfriend]

Fixes

Fixes #1041 by @AetherUnbound

Description

The cause of the test failure described in the issue is the dead link filtering. Luckily, for basically all tests and local applications, we do not need to filter dead links by default. Enabling the default to be modified by an environment variable is the quickest solution for this that allows us to clean up tests that were explicitly disabling the dead link filtering, as a bonus.

The default in settings.py is True, which matches the original setting (before the variable existed, before this PR), so there will not need to be additional changes to the deployment configuration to keep the behaviour the same in staging and production.

Testing Instructions

Run the tests locally and confirm they run. Also confirm they pass in CI.

Checklist

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[github-actions]

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[AetherUnbound]

Allowing this to be set wholesale by env var is a great idea! Tests passed on my machine and code looks good, thanks for the quick fix 🚀

[dhruvkb]

This makes sense to me! 👍

[stacimc]

Very smart and quick fix for the testing issue 🚀