Render comments in Ruby instead of JavaScript

Instead of fetching new comments as JSON and rendering them in
JavaScript, we now return fully rendered HTML. This will let us do
fancier stuff with comment bodies - autolinking, markdown, emoji, etc.

This approach utilises the SJR technique as described here:
https://signalvnoise.com/posts/3697-server-generated-javascript-responses

SJR is somewhat controversial, but I think it's appropriate here.
Writing JavaScript by hand to transform JSON into HTML, as we were doing
before, is much more likely to cause security issues than using SJR.
Render incident comments during initial page load

Instead of polling as soon as the page finishes loading, include the
existing comments in the HTML directly.
Merge pull request #33 from intercom/elk/comments_sjr

Render comments in Ruby instead of JavaScript