Remove legacy topical events [WHIT-3059]

[ChrisBAshton]

Audit

The audit of existing tests for topical events led to three other housekeeping / preparatory PRs:

• Remove unused (legacy) topical event code [WHIT-3059] #11201
• Refactor topical event dependency tests [WHIT-3059] #11202
• Tweak opting into topical events [WHIT-3126] #11216

In addition we've identified the following legacy behaviour and remediation steps:

• Organisations can only be tagged to so-called 'active' topical events (where an end date has been specified, and it is before 'today'). We're happy dropping this and letting organisations tag to any topical event.
• We surfaced 'number of detailed guides tagged to the topical event' on the topical event index page and summary page. We're happy dropping this as the info is available on the Features tab. We proposed removing this a while ago. It looks like the surfacing was first introduced way back in
  46a7798#diff-aebdda5489e75983ed804d3897bf708fadde13cd52b6815c31d25ddbf0254aaeR55-R56, with no clear reason for it
• republish_feature_organisations_to_publishing_api not needed - it happens automatically now through link expansion 🎉
• Some file caching of topical events, but we looked into it and decided the complexity wasn't worth the negligible performance improvement
• We've agreed having start/end date fields offers little value and these won't be carried over to the new style topical events
• "Topical Event Featurings" .feature file (and others like it) have StandardEdition equivalents already

What

The purpose of this PR is to audit the existing legacy topical event code to ensure all functionality has been considered and either carried over or deliberately omitted. The aim is to end up with a draft PR that deletes all legacy topical event code, and a passing CI build. This should make legacy topical event code deletion much smoother.

NB, we'd then need to raise a PR to drop the following database tables:

• topical_events
• topical_event_organisations
• topical_event_memberships
• topical_event_featurings
• topical_event_featuring_image_data
• topical_event_about_pages

...and drop topical_event_id from the features table.

Their behaviour would now be encapsulated in the "editions" table and "edition_links" and "feature_lists"/"features" tables (and whatever tables we introduce for multi-page architecture).

Why

Jira: https://gov-uk.atlassian.net/browse/WHIT-3059

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