Recommend & add official support for django-sendfile2 in place of django-sendfile (custom serve methods)

Is your proposal related to a problem?

• Wagtail provides documentation that recommends django-sendfile which is no longer actively maintained - https://github.com/johnsensible/django-sendfile/issues/74

Describe the solution you’d like

1. Unit test updates - current unit tests explicitly include django-sendfile to support the documented approach it seems, plus probably a bit of a pre 1.0 legacy
2. Documentation updates - documentation advises using django-sendfile when it is probably better to support the more maintained django-sendfile2, it should be directly compatible but this change should come after the unit test changes (or in the same PR)
3. Potentially replace utils/sendfile with an optional import of django-sendfile2 if needed (which is a copy of an even older version of the django-sendfile code)

Describe alternatives you’ve considered

• Do nothing, the current solution of a copy of django-sendfile mostly works but has some issues and will likely break in future Django versions.

Additional context

• Wagtail included django-sendfile for custom serve methods of images, but then moved to a more generic approach around 1.0 https://github.com/wagtail/wagtail/pull/1363
• Our documentation does say django-sendfile but it is not actually used directly, but to be fair it is a bit confusing and maybe something could be clearer.
• Images - https://docs.wagtail.org/en/stable/advanced_topics/images/image_serve_view.html#integration-with-django-sendfile SENDFILE_BACKEND setting
• Docs - https://docs.wagtail.org/en/stable/reference/settings.html#wagtaildocs-serve-method WAGTAILDOCS_SERVE_METHOD setting
• https://django-downloadview.readthedocs.io/en/latest/django-sendfile.html

[lb-]

Removing ‘good first issue’ label as this will likely be a solid amount of work and require someone with knowledge of custom serve methods and django-sendfile to understand.

[lb-]

@spapas please review the updated description and confirm this covers what you are expecting.

Note that the issue templates really do help when requesting enhancements.