Wagtail 5.2 Support

[jams2]

Adds support for Wagtail 5.2. Between 5.1 and 5.2 the wagtailsnippets/snippets/index.html template diverges in a way that maintaining compatibility for both versions would be a burden, so I propose releasing the next wagtail-airtable version without support for Wagtail < 5.2 (suggested by @zerolab).

Changes:

• use wagtailsnippets/snippets/index.html instead of wagtailsnippets/snippets/type_index.html (removed)
• update long-running buttons to use new styles and stimulus.js declarations
• add a redirect back to the snippet list view when you're importing airtable data from the snippet list view - I think this is a better user experience than being redirected to the airtable import view.

[gasman]

FYI, I've opened https://github.com/wagtail/wagtail/issues/11275 to hopefully make the template override less brittle in future :-)