New styling for commenting

[kaedroho]

Adds support for jacobtoppm/wagtail-comment-frontend#11

[squash-labs]

Manage this branch in Squash

Test this branch here: https://featurecommenting-new-styling-o8ko9.squash.io

[jacobtoppm]

Generally looking great, but 2 issues right now:

• Adding a new reply causes no avatar to show on the added reply (all other avatars on existing replies display fine)
• On Chrome, at various levels of zoom, the circles surrounding the avatar seem to have gaps in some places between the avatar and the border, which look a little odd. The border itself also looks a little pixellated - any way we could smooth this a little more? (This is a minor visual bug, but thought I'd mention)