added release_date to job_offer
adapted job_offer show
added data_migration for missing release_dates
Merge remote-tracking branch 'origin/develop' into feature/release-date

Conflicts:
	app/views/job_offers/show.html.erb
	db/schema.rb
	db/seeds.rb
	spec/controllers/job_offers_controller_spec.rb
Merge remote-tracking branch 'origin/develop' into feature/release-date

Conflicts:
	db/schema.rb
Merge pull request #504 from hpi-swt2/feature/release-date

Feature/release date