Prefix the database name used as class name on the document body to avoid conflict with other classes

We do it already on xwiki.org, since for example the "code" database name collides with classes for the code macro and makes the wiki display very bad.

With a prefix such as "wiki-" we would be much safer. (Are there scenarios where '-' is allowed in database name ?).

We would have to document this in release notes, since it might be possible people used that class to skin differently wikis on a same farm (like we use to do on xwiki.org for example).