Big code review of upsert API and Disqus import.

Also:
- Fix: Properly update url path, if changing category slug (instead of
  error message).
- UX: Groups page help text.
- UX: Delay Unhide tips.
- UX: Use CSS to append the guest '?' mark, so can be removed.
- UX: More color differences in the admin intro tour, makes it simpler
  to see where to click?
- Less disk space quota, for blogs, than for forums (like, 10%).