Add Wagtail 4 support

[Tijani-Dia]

We've recently upgraded one of our projects which uses this package to Wagtail 4 but we're getting errors with 2fa when logging in to the admin. It looks like it may be related to this line here: https://github.com/labd/wagtail-2fa/blob/0b2b6e77b3e11a7c282db5f72c18de5692dfaef8/src/wagtail_2fa/templates/wagtail_2fa/otp_form.html#L9.

Thanks a lot for this useful package by the way.

[zerolab]

Looks like this was fixed via #195, #196 (and add some tweaks in #203), so it can be closed