Use monospace format instead of the limited code macro to convert the <code> XHTML tag with complex content

Converting <code> to the code macro sometimes lead to incorrect results, for example when there are links inside the <code> macro. Converting to 

##content##

instead of

{{code language="none"}}
content
{{/code}}

addresses this limitation.

This should only be done for code tags with complex content, because the monospace formatting is technically not the right conversion for code, a code macro is more suitable for semantic and styling reasons. See https://jira.xwiki.org/browse/CONFLUENCE-120 and https://jira.xwiki.org/browse/CONFLUENCE-56.