Added setting to disable find courses links
Renamed setting ENABLE_STUDENT_SELF_ENROLLMENT to COURSES_ARE_BROWSABLE
If setting COURSES_ARE_BROWSABLE is off return 404 on /courses
Removed hardcoded case for edge for /courses
Updated copy in dashboard
Merge pull request #1073 from edx/usman/lms1026-find-courses-link-optional

Added setting to disable 'find courses' links