started to enforce roles #75

detail:
admin now can do everything on the job offers show page
all staff members of the chair can edit/update/destroy job_offers
only the responsible user can accept applications (adapted tests in job_offer_pages_spec for that)
only staff members + admins can create job offers, students can’t
you can only edit your own profile (+ test), an admin can edit every profile #75
students can’t create job offers via “/new” url parameter anymore
added missing login in test #75
students can’t view the overview of students and research assistants, #75
renames "research_assistant" to "staff"

(NOTE you now have to use "staff_index_path" to get to the staff index page)
removes accidentally commited backup files for vagrant
added some test cases for role specific behaviour on job offers show pages #75
Merge branch 'develop-ja-enforce-roles' of https://github.com/tkowark/hpi-hiwi-portal into develop-ja-enforce-roles
Merge remote-tracking branch 'origin/develop-ja' into develop-ja-enforce-roles

Conflicts:
	app/controllers/staff_controller.rb
	app/helpers/users_helper.rb
	app/views/research_assistants/edit.html.erb
	app/views/staff/_form.html.erb
	app/views/staff/index.html.erb
	app/views/staff/show.html.erb
	spec/controllers/research_assistants_controller_spec.rb
	spec/controllers/users_controller_spec.rb
admins can view students overview and staff overview, staff can only view the students overview #75
if you applied as a student but switched to staff afterwards the user link in the job applications list now points to the correct staff page, if you're admin and no longer got a profile the row is non clickable #75