Put inactive students in a separate tab

This partially solves #1467, which adds a new tab under the group view
just for inactive students, resulting 3 disjoint sets:

- { assigned students }
- { unassigned students }
- { inactive students }

And the union of them is the set of all students.

Tested:
- rake test:units
- rake test:functionals
- rails s
- en, fr, pt
- Ruby 1.9.3-p545, 2.1.2
Merge pull request #1510 from zhangsu/inactive_students_tab

Put inactive students in a separate tab