This way we can simplify the HTML and easily apply font awesome icons to
any element.
Note the mixin uses `extend`, which we generally try to avoid. It's OK
in this case, since `fa-` classes only have one rule, affecting the
content of its `::before` pseudo-element. Unfortunately we can't use
`include fa-content($fa-var-#{$icon})` because it's not valid SCSS. We
could make the mixin accept an icon instead of an icon name, and call it
using `has-fa-icon(r, $fa-var-plus-square)`. However, IMHO that would
make the code a bit more complex with no real benefit.
Extract the needed portion of code to a new partial to be able to update
only the elements needed when a new comment is added keeping UI properly
updated.
