removed unnecessary employer selection when creating a new job offer for #134
some more refactoring
added external employer listing for #181
added possibility to create external employers through the GUI
updated seed file
refactored student and staff profile into own user_profile partial
removed trailing whitespaces
some more minor refactorings
fixed bug in users helper [ci skip]
removed unnecessary helpers [ci skip]
Improved ability.rb and some general refactroings
Merge branch 'develop-ak-cancan' into develop-ak

Conflicts:
	app/models/ability.rb
	app/models/employer.rb
	app/models/user.rb
	app/views/employers/_form.html.erb
	app/views/employers/show.html.erb
	config/locales/views/employers/en.yml
deleted some unused files
Merge pull request #225 from tkowark/develop-ak

Refactorings and some Abilities