Add generic field visibility for StreamField StructBlocks

[woodwoerk]

Add a generic pattern for showing/ hiding fields within a StructBlock in Wagtail streamfields based on the value of a sibling ChoiceBlock field.

Purpose

When a StructBlock has fields that are only relevant for certain choices (e.g. layout-specific settings), the admin shows all fields unconditionally. This change aims to improve the user experience by only showing necessary and relevant fields.

Solution (summary by Claude)

blocks/conditional_struct_block.py — A ConditionalStructBlock base class. Subclasses declare conditional_rules at class level:

class MyBlock(ConditionalStructBlock):
    conditional_rules = [
        ConditionalFieldRule(trigger='layout', target='extra_settings', show_for=['advanced']),
    ]

Subclasses with non-empty rules are auto-registered via init_subclass. get_conditional_rules_config() serialises all registered rules as a JSON object keyed by block fingerprint — the sorted set of the block's declared field names, which uniquely identifies the block type in the DOM without requiring Wagtail internals changes.

static/kausal_common/js/conditional_struct_block.js — A single generic MutationObserver script that reads window.KAUSAL_CONDITIONAL_RULES and sets up show/hide listeners on any matching .struct-block using data-contentpath selectors. The fingerprint is computed from direct children only to avoid false matches from nested struct-blocks.

wagtail_hooks.py — An insert_editor_js hook that injects the rules config as an inline <script> followed by the JS file. Because kausal_common is in INSTALLED_APPS, the hook is discovered automatically; no registration is needed in consuming apps.

Usage
Extend ConditionalStructBlock instead of blocks.StructBlock and declare conditional_rules.

---

✅ Pre-Merge Checklist

Type of Change

• Set the PR's label to match the nature of this change

Testing

• Built Unit tests (unit tests added/updated)
• Built E2E tests (if applicable. E2E tests added/updated)
• Authorization is tested (permissions and access controls verified)
• Manually tested locally in all affected projects (functionality verified)

  Manual testing instructions

  If feature requires manual testing by reviewer, you can provide instructions here.

Internationalization & Accessibility

• New strings are translatable (all user-facing text uses i18n)
• Accessibility standards met (WCAG compliance, screen reader support)

Dependencies

• Dependencies are merged (if applicable. If the change depends on other PRs)

[woodwoerk]

@tituomin The fingerprinting approach to associate Wagtail generated blocks with client-side JS feels a bit flaky, there might be another way to handle that