Add Codemirror editor

see http://beta.woltlab.com/index.php/Thread/558-ACE-Editor-im-ACP-einbinden/
Merge branch 'master' into codemirror

Conflicts:
	wcfsetup/install/files/acp/templates/styleAdd.tpl
Add codemirror LICENSE and README
Remove modes that are unlikely to be used
Remove keymaps and html files from codemirror
Merge pull request #1333 from WoltLab/codemirror

Add Codemirror editor