Wagtail 2 fixes

[philgyford]

Some fixes to get this working on Wagtail 2 (I'm using Wagtail 2.4 and Django 2.1).

• Fix imports that broke from Django 2.0+ (the fixes should still work with older versions of Django)
• Fix imports that broke from Wagtail 2.0+ (same)
• Replace broken use of gravatar template tag (which is not included) with xtd_comment_gravtar_url
• Fix layout of avatars in Wagtail admin (they were too big for current Wagtail)

Also:

• Allow the use of a custom django-comments-xtd comment model using the COMMENTS_XTD_MODEL setting
• Use relative dates for the list of comments in Wagtail admin, to be consistent with other admin lists

[philgyford]

This continues to work with Wagtail 2.5 and Django 2.2.