CRAFTERCMS-202: Multiple forms open for the same content

While a form is open, clicking to edit the same content must not open
another form (window) -even if the form is reloaded.
CRAFTERCMS-210: Each RTE makes a call to /site/sitename

If there are no configured stylesheets, each RTE makes a call to
/site/sitename which results in a 500.
Merge branch 'master' of https://github.com/quiaro/studio
Merge remote-tracking branch 'david/master'