Previously, the comment reply form was being created entirely
client-side by cloning and modifying a <template>. This was nice because
it meant that a network request wasn't necessary to display the form,
but it also had downsides.

For example, if a topic was locked after a user had already loaded the
page (or their notifications page with a comment from that topic), they
would still be able to click Reply and type in a comment, and wouldn't
know that replying wasn't possible until they actually tried to submit
the comment.

By switching to using intercooler for this form, we can do server-side
validation to check permissions before showing the form, and it also
simplifies some other aspects, such as the warning about replying to an
old comment, which previously needed a data-js-old-warning-age attribute
in the HTML, but is now just part of generating the reply form template
server-side.