Remap section key ('§') to escape

My British keyboard layout 11" MacBook Air has a tiny escape key and a
useless section key directly below it.
Configure Airline compatibility with other plugins
Configure YouCompleteMe
Configure UltiSnips

We need to redefine UltiSnips's triggers so as not to conflict
with YouCompleteMe.
Replace tmux status line conf. with generated Tmuxline conf.