Add subject and body to mailto links

[TopDevPros]

Fixes #5950. Updated code from PR #6451 which was abandoned 3 years ago. Added more comprehensive tests.

[Y] Do the tests still pass?
[Y] Does the code comply with the style guide?
    [Y] Run make lint from the Wagtail root.
[Y] For Python changes: Have you added tests to cover the new/fixed behaviour?
[Y] For front-end changes: Did you test on all of Wagtail’s supported environments?
    Chromium and Firefox on linux
[N] For new features: Has the documentation been updated accordingly?

    The only possibly related docs are in wagtail/docs/extending/rich_text_internals.md. 
    It appears that we would unnecessarily complicate the current docs with details about
    the new optional fields.

[squash-labs]

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Test this branch here: https://topdevpros5950-au4a8.squash.io

[lb-]

This works well, thank you. Thanks also for adding robust tests.

I have one line of feedback, but can add this easily in the PR when merging.

When you save an email address, then edit, then switch to a non-email field, you get the raw link in the other sections. I think this is fine but just calling this out. I see that we do this for the reverse (kind of), when a non-email link has been added.

No action needed on this, just leaving notes for future self if we want to change this behaviour one day.

[lb-]

Can we please make this work as a multi-line field, something like...

Suggested change

	body = forms.CharField(required=False)
	body = forms.CharField(required=False, widget=forms.Textarea(attrs={"rows": 3}))

[lb-]

Rebased/formatted files, added the textarea widget and pushed. Will review the CI and merge in if all good.

[TopDevPros]

@lb, thank you!