Partial implementation for ticket #2442: MobileProfile plugin should allow manual...

Partial implementation for ticket #2442: MobileProfile plugin should allow manual switching between regular and mobile rendering modes

http://status.net/open-source/issues/2442

Notes:
* Mapstraction causes JavaScript errors in XHTML mode, breaking our code if we're run later so the link doesn't work to get back to Desktop.
* not 100% sure how safe feature detection is here?
* Currently will be useless but visible links if no JS available; need to fall back to server-side for limited browsers