Remove prototype javascript files.

We don't seem to be using them, we seem to be using jquery.
Add unobtrusive javascript gem.

This is to provide the js side of Rails built-in helpers using the
jquery framework rather than prototype.
Include unobtrusive js in the admin layout

We use the :confirm option of the submit_tag helper. Fixes #1042.
Add confirmation to outgoing messsage destroy.

Closes #1053.
Merge branch 'feature/restore-unobtrusive-js' of ssh://git.mysociety.org/data/git/public/alaveteli into feature/restore-unobtrusive-js