Support Wagtail 6 + add sharing link to Action Menu

This PR is based on our fork of the project and I'm very happy to offer our changes back upstream.

The changeset:

adds Wagtail 6 / Stimulus JS support to wagtaildraftsharing
adds an additional way to generate a sharing link, from the Action Menu at the foot of the page - if there is an unpublished Draft only
makes some of the wording around 'draftsharing' customisable, in a way that won't trigger migrations if a project does use non-default wording
expands test coverage, bringing things up to ~98% and effectively 100%
adds CI using tox and Github Actions
sketches in a Release GHA which can be uncommented for use on this upstream repo
crops Python 3.8 support (which is now EOL), adds 3.9, 3.10, 3.11 and 3.12 to tox.

For more details, please see the PR on our side: https://github.com/mozmeao/wagtaildraftsharing/pull/1/

Note that this hasn't been tested in production yet, so feel free to wait a few weeks before merging. Equally, if during review you spot something that could be done better, please feel free to mention it!

torchbox / wagtaildraftsharing

Support Wagtail 6 + add sharing link to Action Menu #3