fix ace editor Syntax Highlighter mode load 404

Created by: tsl0922

This error happens on production mode only, when you edit a markdown file, the content can't be correctly highlighted cause of a 404 error of ajax loading mode-markdown.js.

Ace load js use a relative path to itself and our ace.js is packed to application.js, so the final url ace used to load js is error.

I use nginx gzip_static module, so it's better to precompile all ace related js:

config.assets.precompile += %w( ace-src-noconflict/* )