This commit mirrors the current functionality for Wagtail 5 in Wagtail 6 by
creating a Stimulus JS controller and hooking it into Wagtail in a similar
way as for Wagtail 5.

For the page history view, the HTML templating is different, so the previous
approach overrode wagtailadmin/pages/revisions/_actions.html to slot in custom
markup that the JS could hook into. For Wagtail 6 we do something similar,
extending wagtailadmin/generic/history/action_cell.html to slot in a bit of
custom markup.

For snippet-list view, we update the HTML output for the share_url method
to hook in the Stimulus controller used above.

The JS in the Stimulus controller is deliberately very similar to the
original JS. I wondered about refactoring out the main duplicated code
(the main function which triggers the creation of new links via a POST
to the backend, and the function which upgrades the View button on the
list view of existing sharing links to be a clipboard link). However,
for now - and with a view potentially to deprecating pre-Wagtail 6 behaviour
- I've gone with some repetition rather than DRY code.

Note that we don't load both JS files into the browser - we only load
the appropriate one based on the version of Wagtail in use

...if there is an unpublished draft that can be shared

…uring link creation

…more tests

Bit of a nitpick here, I know - sorry!

incl datetime_now -> timezone_now to avoid mis-assumption about which library it's from

… tidying earlier

Brings coverage up to 98%, and effectively all code that needs coverage

…was coming back as a naive datetime

So we patch it in the tests and warn about it

Note the tolerance of a should-not-build version python3.12-django3.2-wagtail6.1 - may be there are better ways around that

No need to have three things going when ruff does the lot

… expiry timestamps

Note the addition to the README, requiring the use of settings.USE_TZ=True