Previously, an entry would be added to the topic log even if someone
didn't actually change the topic's tags at all. This commit fixes that,
as well as moving the the template fragment that renders the list into a
separate file in includes/ that can be used both for the initial render
as well as by intercooler when it updates that section of the page.
This was a bit confusing with the topic icons using a dashed blue border
for "missing icon". Now that the "already used" buttons are far more
distinct, this isn't necessary any more anyway.