This makes API endpoint for Commit Statuses return empty array instead
of 404 when commit exists, but has no statuses.

Closes #3080

This MR simplifies CI permission model:
- read_build: allows to read a list of builds, artifacts and trace
- update_build: allows to cancel and retry builds
- create_build: allows to create builds from gitlab-ci.yml (not yet implemented)
- admin_build: allows to manage triggers, runners and variables
- read_commit_status: allows to read a list of commit statuses (including the overall of builds)
- create_commit_status: allows to create a new commit status using API

Remove all extra methods to manage permission.
Made all controllers to use explicitly the new permissions.