Quizzes Visual Design & Quiz Attempts

* Quiz Creation
  * Show popover if a user tries to delete a problem when it is the only problem it in quiz
  * Show error messages for which areas need to be fixed
  * Shows a general error if anything is wrong
  * Redirects to quiz page instead of showing success message
  * Some more tests

* Quiz Taking
  * Users logged in can see quiz pages, otherwise redirected
  * Users who are not enrolled in the corresponding course **cannot** take a quiz
  * Tests for Quiz Attempts

* Removed unused google fonts

* Viewing lessons has a minor change. Instead of using a `p` tag, it uses a `div`.

@abboudm @DavidMah