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
Remove redundant comments with hash rocket

The comment comments out a line of old code. This should generally not
be left in the codebase as the code can be retrieved using source
control.
Replace forgotten symbol key hash rockets

The symbol key hash rockets in the controller and routes.rb are somehow
missed by the `hash_rocket` gem. They got replaced, however, when
rerunning `hash_rocket`.

The Rake task files have a different suffix, which are missed when
running `hash_rocket`.
Replace symbol key hash rockets in comments

These code snippets are meaningful to be left in comments.
Replace symbol hash rocket in embbeded ERB string
Simplest way of hiding marked criteria
...or just do the AJAX calls instead.
Merge pull request #1510 from zhangsu/inactive_students_tab

Put inactive students in a separate tab
Merge pull request #1521 from zhangsu/remaining_hash_rockets

Replace or remove remaining hash rockets
Merge pull request #1524 from arkon94/collapse

Hide unmarked criteria. Fixes #61.
Fix for issue 1455: Added check for student wanting to work alone before displaying invite button.
The button is disabled when group max is reached.
Merge pull request #1525 from ealonas/issue-1455

Fix for issue 1455: Students cannot access other students repos anymore. Fixes #1455