Adopt core shared header for Snippet type listing header

Is your proposal related to a problem?

The Snippet type listing header is one of the last major header usages within the admin that is not using the using the core (non-slim) shared header template or include.

Describe the solution you’d like

• Using either a template that extends the shared header OR the template include, ensure that the wagtail/snippets/templates/wagtailsnippets/snippets/type_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.

Describe alternatives you’ve considered

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

Additional context

• See initial work to unify headers https://github.com/wagtail/wagtail/issues/8539
• See a very similar work for the model admin header - https://github.com/wagtail/wagtail/issues/8907 (it may make sense to do that first as some blocks/args will be required for that which could be used for this issue)

Notes for potential implementation approach

wagtail/snippets/templates/wagtailsnippets/snippets/type_index.html

• Firstly, you will likely need to create a new template that extends OR includes the header.
• Firstly, set up a fragment that uses {% include 'wagtailsnippets/snippets/headers/list_header.html' %} (this should be just breadcrumbs) and pass it into the block breadcrumbs (be careful here as it may not render in the same way, investigate this usage carefully)
• Pass in the icon name snippet instead of rendering it on its own
• model_opts.verbose_name_plural|capfirst becomes the title arg
• The search URL looks like a copy verbatim of the existing search in the shared template
• {% if locales %} - this may not have a home (adds an extra col), see if there is a nicer way to achieve this goal
• If we are happy to reverse the order, the locale selector can go into extra_actions and the action_url should be sufficient for can_add_snippet

[lb-]

@akshay-gupta7 no problems at all & no rush. Just add a comment to this issue if you’re stuck with anything or have any questions.

[akshay-gupta7]

Appreciate the support @lb- !