Add AJAX posting of comments

Pull comments JS out of mapFeature.js into comments.js

Also moves comment template section into a partial for readability

In order to avoiding having to changing core parts of the Django comments
app or bringing in another commenting library like django-fluent-comments,
this gets the *entire* plot detail page when adding a comment, and then
strips out the comment section from it and replaces that HTML content.
@steventlamb I was on the fence about pulling the comments HTML section out at all.

I initially thought it would be necessary to return HTML partials, but in the end had trouble modifying the existing comment endpoints.

I figured I'd leave it in a separate HTML file for readability, but if it's causing confusion maybe I should just revert it.