Adds two separate user settings. One only affects "external" links (the
actual links for link topics), while the other will also affect links to
comments pages (including text topics).
When on mobile, it wasn't previously possible to tell whether you had
any notifications/messages or not without opening the sidebar to check.
This adds a small "badge" to the sidebar button when the user has
notifications, showing them how many unread items they have.
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.
