Attendance endpoint inconsistencies

[smartspot2]

The endpoint for mentors to get attendances is /api/sections/*/attendance/, but the endpoint for students to get attendances is /api/students/*/attendances/. In particular, the endpoint for mentors is singular, while the endpoint for students is plural.

It would make more sense if both endpoints were singular or both endpoints were plural; personally, plural makes a little more sense, since we're always getting all of the attendances (i.e. all attendances associated with the section, or all attendances associated with the student). Open to arguments for swapping to singular though.