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

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

[x] My pull request has a descriptive title (not a vague title like Update index.md).
[x] My pull request targets the default branch of the repository (main) or a parent feature branch.
[x] My commit messages follow best practices.
[x] My code follows the established code style of the repository.
[x] I added or updated tests for the changes I made (if applicable).
[N/A] I added or updated documentation (if applicable).
[x] I tried running the project locally and verified that there are no visible errors.

Developer Certificate of Origin

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