Move job position data to a data file

This moves job position data to the data/jobs.yml file.

This lets us do cool things like show an "Apply" button on the detail page for the job when we're actively hiring for it. See https://gitlab.com/gitlab-com/www-gitlab-com/issues/698 and https://gitlab.com/gitlab-com/www-gitlab-com/merge_requests/2226.

Closed position:

Open position: