Frontend: Position synset tooltips nicely at side of hovered synset

This is done by listening on onmouseenter events on the synset-DIVs.

Also the alternating background of the synsets needed to be rewritten for this.
 IMPORTANT: You may need to update the Python Package Jinja2 to version, which does the rendering for us. We need to use the 'loop.depth' method, which just recently got introduced. If you have PIP installed, just run "pip install Jinja2 --upgrade".