Implement CodeMirror instead of Bitter for syntax hightlighting

[brendo]

Originally from this issue, Bitter preprocessing the XML at the server level which causes the page to hang and take a long time to execute.

By moving the processing off the server and into the client side, things should be quicker.

Implementation details of CodeMirror start in this comment

[brendo]

Note that the CodeMirror submodule would enable the rest of Symphony to use the syntax highlighting if required (ie. we have the current highlighting for error pages, but if CodeMirror is around, it is enhanced).

[nitriques]

It's much faster now since b3aabb7

I think it can even be closed.