Fix JavaScript submitlink on "Add submission"

Template admin/newsletter/submission/change_form.htm always tried to add
some JavaScript event for submitlink, even on "Add submission" page
where there's no submitlink.

Fixed by porting submit_interface.js to jQuery.
Merge pull request #68 from jkuczm/fix_admin_submission_template

Fix JavaScript error on "Add submission" page, updated to use jQuery.