Secure submissions app setting (Preview only!)

This activates submitting with authentication, which will be noted on the submission. (There's still no UI for making a domain _require_ authentication though.)

What you see if you're a previewer (@dimagi.com):
![screen shot 2013-09-12 at 11 53 04 am](https://f.cloud.github.com/assets/137212/1132003/7385f57a-1bc3-11e3-9c4d-bb3a8c86e154.png)
What you see otherwise:
![screen shot 2013-09-12 at 11 53 43 am](https://f.cloud.github.com/assets/137212/1132011/8364e942-1bc3-11e3-9f90-f4fccd7af49d.png)