Add Rich Text Editor 

Replaces NB's <code>textarea</code> with a Rich Text Editor, using the CKEDITOR library. Functionality includes Math Formula Formatting and Code Syntax Highlighting. 
If we don't do anything, it will cause issues for comments that contain the 3 following characters: 
& 
<
>
Unless CKEditor escapes those characters, but if it is so, then we'll have at least 3 issues to solve: 
- How does CKEditor escape those characters ? We'll then need to escape every previous comment in the database accordingly...
- If we escape certain characters, does it mean that we can't use a textbox to annotate ? 
- What happens when using the comments in order to generate documents that aren't HTML, for instance annotated PDFs: it works great right now because nothing in the databased is being escaped, so we store exactly what the author typed.