Switch order of html/text for StatusNet backwards compatibility

Switch order of html/text for StatusNet backwards compatibility

StatusNet chooses the first content element in an Atom feed, while
it should really choose the 'html' representation for its 'rendered'
and 'text' representation for the (text-only) 'content'.

GNU social will implement a better algorithm for retrieving Atom
feeds, but that is yet to be done. So to avoid having link-less posts
on remote nodes, we'll just do the old switch-a-roo.

Other Atom readers, such as Mozilla Firefox, has the reverse priority
(choosing the last of the content elements).