Fixes #314. Fix XSS vulnerabilities in web client code

The main place we were vulnerable was in girder.confirm, where
we were injecting html based on user-supplied content. I was able
to exploit this successfully, but have now patched girder.confirm
so that its default behavior uses text() rather than html(), and
this can only be overridden by users who explicitly acknowledge
that they have escaped any user-supplied data being passed in.