1.delete non-existent css reference in both studentCourseJoinConfirmation.jsp & instructorCourseJoinConfirmation.jsp
2.remove wrong padding style for Confirm and Cancel buttons in instructorCourseJoinConfirmation.jsp
Merge branch 'master' into Issue2134
[2134] *joinConfirmation.jsp refers to a *joinConfirmation.css

fixes #2134