Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

- Remove all view related methods from Ci::Build and CommitStatus
- Remove unused Ci::Commit and Ci::Build methods
- Use polymorphism to render different types of CommitStatus

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.

`artifacts?` method checks if artifacts archive is available.