fix(app): missing event.preventDefault

[corburn]

This PR adds event.preventDefault() to the $on('$stateChangeStart) Auth intercept because it fixed an intermittent bug in my code.

I made the following change in my code:

// Before:
if (next.authenticate && !loggedIn) {
    $location.path('/login');
}

// After
if (!loggedIn && next.name !== 'login') {
      event.preventDefault();
      $state.go('login');
}

Before adding event.preventDefault(), the state intercept would intermittently fail to redirect the user to the login view. Specifically when clicking links. Adding event.preventDefault() fixed the problem.