- Drop /usr/share links which were causing different behavior
  locally for me than for (almost) anybody else.
  Specifically, I was getting mathjax 2.1; latest is 2.2 which seems to
  to change display math alignment to centered.
- Upgrade CodeMirror submodule to 3.16.
- Upgrade jQuery CDN link 1.10.1 -> 1.10.2.
- Tiny html5 cleanups.