Fix merge conflict.

TODO

src/backend/__init__.py

src/backend/backend.py

src/frontend/css/editor.css

src/frontend/js/editor.js

src/frontend/tpl/editor.html

src/parser.py

src/validation.py

static/build/external/jquery

static/external/codemirror2

static/external/mathjax

static/frontend/css

static/frontend/js

static/img/cross.png

static/img/info.png

static/img/load.gif

static/img/tick.png

tests/parser.py