API: Subsite support for menu of cms (hides admins that don't declare support) (fixes #101 and #89, )

* Hide admins without subsite support from subsites menu
* Add subsite support to default site areas
* Enable reloading of subsites switcher dropdown when navigating the
site, and when editing subsite areas

Also relates to #85 as it now seems more appropriate to do a hard refresh when changing sites (at least between main and subsites).

Tested on IE8, Chrome, and Firefox