(bug 27316) Add option for (en|dis)abling multiple-file uploads

In addition to disallowing multiple file selection, we can now
disallow multiple files being uploaded in a single session through
a single configuration variable. This is used not only to disable
the creation of new file inputs and buttons, but also to disable
the multiple file select functionality if you've decided to leave
it on.

Change-Id: I694e7ab1f2dec461e781f6ebf672a66cd2ee12b5
Merge "(bug 27316) Add option for (en|dis)abling multiple-file uploads"