handle empty sections for section enrollment

before, if a section from the active semester had no enrollment, it
wasn’t added to the cache and thus had to be looked up individually
even though we already know that there is no enrollment in that
section. So added a chunk to mark the section as empty when it’s from
the active semester but no enrollments are available from /students
remove single quotes around normal strings

repr(‘some string’) goes out to “ ‘some string’ “, not “some string”.
So, if a string is passed as the first argument, just use that.
overhaul logic for match_section and make tests

match_section is officially done. See doctoring for details on params
and capability