chore: Improve test coverage

[mgax]

Fix #378.

• Refactor the existing tests to use wagtail-factories and clean them up a bit.
• Fix an annoying issue where running tests locally would pollute the shared memcache and break the generation of page URLs for the development instance.
• Remove some unused or overly complicated code (e.g. the OrderedSet implementation).
• Add tests for previously untested areas of the code (e.g. bibliography, snippets, cache purging, the megamenu).
• Fix a couple of subtle bugs:
  • The blog redirect to IESG statements would format the date_to URL parameter incorrectly.
  • The megamenu URLs for direct children of the menu item's page would be generated incorrectly.
• Mark some bits of the code as # pragma: no cover because they are impractical / not useful to test.

[mgax]

I've pushed an update to cover IAB-specific pages too.

[SharmaineLim]

Hey Alex, just have a few comments, and something to consider for the tests.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.89%. Comparing base (4ad3143) to head (1da2d8b).
Report is 7 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #395       +/-   ##
===========================================
+ Coverage   82.72%   95.89%   +13.16%     
===========================================
  Files         124      143       +19     
  Lines        2599     3117      +518     
===========================================
+ Hits         2150     2989      +839     
+ Misses        449      128      -321     

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[SharmaineLim]

The tests are looking good to me, I just want more documentation. :)

Also flagging a small concern about coupling the BE and FE a bit more tightly with the tests.

[mgax]

@SharmaineLim I've added more docstrings and comments. I've also updated CONTRIBUTING.md to explain the use of pytest.