fixed vote arrows placement in deep comment thread + other things

- fixed bug in deep comment thread where the upvote and downvote arrows
  are placed above the actual comment entry block
- use inline block instead of float for subscribers/online users stats.
  thank you holloh, i really should have listen to you more seriously :)
- added style for disabled submission button (may be we should make this
  generic?)
- force our own sans serif for all buttons
- larger font size for collapsed comment
- added more icons into sprite
- changed icon for gilded submission/comment
- added style for codeblock with line number but it's disabled by
  default. set $show-line-num-in-codeblock to 1 to enable it
- unimportant small markdown changes :)