Remove nowrap and border from <code> elements

This prevents <code> elements pushing out the cells of a table,
but conversely mars the formatting of those elements as it
allows them to wrap. I had to remove the border round <code> 
elements, too, as otherwise the border breaks across lines and 
looks odd.

But it does fix issues where pages from the wiki would result in
content areas which are too wide when proxied by the website.

Fixes XWALK-704