Admins can delete students

Added a route, destroy action, and link in the student/show page to allow AdminUsers to delete students.  Cuke test included.

Tests all ran green for this commit except two SMTP::Authorization errors which I assume are because the gmail login/passwords aren't in the forked repo, so that needs to be checked on heroku or your personal machine.

Also, I came upon a problem in Student#index action where it only displayed students whose archived attribute was false.  The problem is that the default value is nil and only ever gets set to true, so new students weren't being displayed.  Added a migration to make the default false, change all current null values to false, and disallow null values in the archived column..