chore: Improve test coverage

[mgax]

Fix https://github.com/ietf-tools/www/issues/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.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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[mgax]

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