Revert "Replace quickfixsigns with gitgutter."

gitgutter is a bit slow and has redraw flickers.

This reverts commit 5d94b8fa57d6e4f09c5506c73a527ae8b2e428c1.
Remove Syntastic. It's too damn slow.

I couldn't work out how to disable the Haml check.
Add pretty symbols for Quickfixsigns.

Symbols stolen from the README of https://github.com/luxflux/vim-git-inline-diff.