Provisional workaround to sanitize rating comments

This solves (but not in an optimal manner) Issue #318. This problem arises because the jinja2 *automatic autoescape* (flask default) is disabled in the Notebook. The particular Notebook implementation needs this feature disabled in multiple points, despite it is a major security threat. This might be (as usually occurs) related to Issue #319.

This commit only sanitizes the particular point causing the problem, but *automatic autoescape* remains deactivated. Not good. I'm working in a better but more complex solution.