CLC-3528 - Updated user role logic. Added button to Dashboard page

https://jira.ets.berkeley.edu/jira/browse/CLC-3528

* Moves logic for determining if the user should see the 'Create a Course Site' buttons/link to it's own function - canViewAddCourseOption(), and used within replaceStartANewCourseButton() as the first conditional required to continue. The canViewAddCourseOption() logic also updated to include teachers that also have the 'student' role.
* Adds button to the User Dashboard in addition to the Course Index page.