Further problem-desc cleanup.

* Removed all options that can be easily done through HTML: hiding variable names, uses Returns: by default, doesn't resize images, font-size is 16px.
* Merged showDefinition and forceDefLanguage. Although current implementation of this merge was complicated (could use some tweaks in the html renderer so that a `default` setting is possible).
* As a result, there is no CSS in problem-desc-tmpl