Feature/inform

This pull request adds a notification modal if the user trys to book a graduate job even if he is not allowed to. By increasing single_job_offers_requested, the user is allowed to book a single graduate job.
To check if a user got forwarded you can use something like 
 `response.should redirect_to(job_offer_path(@job_offer)) ` 
If you also want to check whether a mail was sent, than use ActionMailer.deliveries and check the count or even the sent template.