Fixed some divs in the basic html template to reproduce previous conversion

Working with reveal template, I found some discrepancies between the previous html references and the obtained by the new jinja converter.
The were also a problem with the output of the cells because the jinja converter generated  some "white" cells (empty) before getting some images or mathjax equations, etc...
I detect that the differences came from the basic_html.tpl...
So, I am proposing some modifications to resolve the problem and getting closer to the previous references.

Discussion welcome :-)
When we resolve this, I can do some css modification to reveal converter to get it right...

Cheers.

Damián.