Template fixes for Wagtail 4.2

[jams2]

Based on https://github.com/wagtail/wagtail-airtable/pull/48

Fix the templates to bring them in line with Wagtail 4.2, and fix broken bulk actions.

With the new button styles, a button with an icon doesn't play well with long-running buttons, so I've changed the "Import foo" button to just use the long-running style.

Manual tests performed for Wagtail 4.2 compatibility:

• [X] Import snippets from Airtable
• [X] Save new snippet to Airtable
• [X] Save new snippet with RevisionMixin to Airtable
• [X] Import snippets from Airtable to snippet with RevisionMixin
  • Import works, but revision not created (will open an issue for this)
• [X] Import pages from Airtable
• [X] Save page to Airtable

[jams2]

I'm not sure why the CI is failing, it looks like the tests are passing.