inbox and miscellaneous little stuffs

- early version of inbox
- convert wiki's toolbar style to %tn-switch-button and apply it on
  inbox's toolbar
- convert comment's bubble style to %tn-comment-bubble and apply it to
  message body in inbox
- move cakeday icon to scss/_icons.scss to make it available in
  subreddit stylesheet too.
- only apply engraved style to .menuarea when there's RES *and* only
  inside .commentarea
- added icon for new message marker in inbox