Adopt core shared header for ModelAdmin headers

[lb-]

Is your proposal related to a problem?

ModelAdmin header is one of the last major header usage within the admin that is not using the using the core (non-slim) shared header template.

Describe the solution you'd like

• Using either a template that extends the shared header OR the template include, ensure that the wagtail/contrib/modeladmin/templates/modeladmin/index.html view uses the shared header.
• This is so that shared styles will all work the same and do not need to be maintained across multiple files.
• Must preserve existing ability to override the modeladmin header and relevant templates to add custom content.

Describe alternatives you've considered

• Leave as is, ok to maintain these in isolation for the most part.

Additional context

• Any blocks must be preserved, especially for modeladmin, as developers will often extend templates to add custom content.
• Split from initial work to unify headers UX Unification - header component with variations #8539

Notes for potential implementation approach

• Most likely need to use template inheritance for this, create a new wagtail/contrib/modeladmin/templates/modeladmin/includes/header.html that extends wagtail/admin/templates/wagtailadmin/shared/header.html
• Add suitable blocks to the shared/header if needed, rough outline below; (note: as this is an extends, we will probably need a with added in the wagtail/contrib/modeladmin/templates/modeladmin/index.html to pass down the right variables.
  • {% block h1 %} inside <div class="col">
  • {% if view.header_icon %} will need to be replace with that icon being passed into the include
  • view.get_page_title will need to be passed into the include
  • self.get_page_subtitle will need to be passed into the include
  • {% include 'modeladmin/includes/result_count.html' %} will need to be passed into the include as description (with a release note change that this now appears outside the h1 tag (which makes sense)
  • {% if search_url %} will need to be wrapped in a block {% block search %}
  • {% block header_extra %} should probably be set up as a fragment and passed into extra_actions on the include (this is a small breaking change, this block will no longer render outside .right column but this should be ok to
• Clean up - we should drop the support of block right_column_classname and ensure the styles still work, remove any now unused styles, add release note about this no longer being supported.

[salty-ivy]

@lb- working on this one first, then followed by #9227 as you suggested.

[salty-ivy]

@lb- PR is Ready.

[lb-]

Thanks. I'll flag it for review.

[lb-]

Will need to raise a new issue after this - the clean up step above and probably should try to align the right/left class names with BEM naming if possible.