tinyMCE: use a bundle with needed plugins

Move the mathelement and epsilonwriter plugins under the TinyMCE js/ dir. Then build the bundle:

grunt bundle:minified --themes=modern --plugins=charmap,image,link,lists,paste,table,textcolor,wordcount,mathmlelements,epsiwriter

Cf repository labnbook-documentation-dev for details.

Closes #247 (closed)