Fix PayPal Standard session loss in WordPress (5.30)

[christianwach]

Overview

Fixes this issue on Lab. See issue for detailed discussion.

Replaces #18502 to merge against 5.30.

Before

PayPal Contribution completes in CiviCRM, but the user is stuck on an error page.

After

PayPal Contribution completes in CiviCRM as expected.

Comments

Other CMSes may still need to implement similar fixes, but can use the supplied CRM_Utils_SameSite class.

[civibot]

(Standard links)

• If this is your first pull-request for CiviCRM, please browse CONTRIBUTING.md for information about the development and testing processes.
• If you are reviewing this pull-request, you may wish to consult the test sites and the Review Standards (long template, short template).

[christianwach]

@eileenmcnaughton I've renamed the methods in this PR - hope they're suitably generic.

[eileenmcnaughton]

@christianwach yes looks good

Test fail feels unrelated

[eileenmcnaughton]

test this please

[eileenmcnaughton]

@kcristiano let me know if OK to merge

[kcristiano]

@eileenmcnaughton I have done r-run in both sandbox mode and live. I used two CiviCRM sites, Firefox and Chrome on Linux, Safari on Mac.
I haven't been able to test all the user agents.

I do think it's better in than out, but I was hoping someone would have the ability to test old user agents.

[kcristiano]

OK I just remembered a server I could run this with IE 11. So I've tested in IE 11 and it looks good.