resolved bug - ability to read instead of show
Merge branch 'develop' into #380_Checkbox_for_student
students only can activate themselves
students can see for students visible students on the students page
#380_Checkbox_for_student
Merge branch 'develop' into #380_Checkbox_for_student
removed unnecessary space
- deleted code, which was commented out
- corrected indentation
#380_Checkbox_for_student
Merge branch 'develop' into #380_Checkbox_for_student

Conflicts:
	app/controllers/students_controller.rb
	app/models/ability.rb

#380_Checkbox_for_student
- more explicitly control of rights
#380_Checkbox_for_student
applied correct scopes for different cases of visibility

#380_Checkbox_for_student
adapted test cases for new visibility statuses
#380_Checkbox_for_student
Merge branch 'develop' into #380_Checkbox_for_student
Merge branch 'develop' into #380_Checkbox_for_student
Merge pull request #505 from hpi-swt2/#380_Checkbox_for_student

#380 checkbox for student