Features cleanup

Merged Achilleas Pipinellis requested to merge features-cleanup into master
  • Remove unused self_gitlab_hosted reference from features.yml
  • Replace not_on_gitlab_com with gitlab_com in features
  • Remove unused gitlab_ce and jira keys from competitors
  • Document all possible attributes of features.yml

Closes https://gitlab.com/gitlab-com/www-gitlab-com/issues/1625