remove redundant `after_sign_in_path_for` method

The `after_sign_in_path_for` method in the ApplicationController was
duplicating the default Devise functionality.  The only different piece
was that we had mapped the users#show action to the named route
'dashboard', whereas Devise will automatically look for a named route
for a resource, '<resource>_root'.  This was the only place
'dashboard_path' was referenced, so removed it and renamed the route to
'user_root'.  So now upon successful authentication, Devise will
redirect to 'user_root_path'.
customize authenticate required message
Use Devise `authenticate_user!` helper method

Removed all custom code that checked for `user_signed_in?` and gave an
error message if you weren't signed in.  `authenticate_user!` will
handle all of that for us now.
Merge pull request #17 from cit-jmu/redirect-to-login-for-auth

Redirect to sign in when authentication is needed