Ci Project migrate

- This doesn't migrate: allow_git_fetch, coverage_regex, timeout. Since this are project configuration settings I would propose to migrate them to `.gitlab-ci.yml`.
- This requires offline migrations.
- It simplifies database models making all CI objects to be attached to: Project.
- It removes Ci::Project, but makes /ci/projects working by adding method: Project.find_by_ci_id for backward compatibility (badges, triggers).
- We should add default `timeout` to Application Settings.
- It misses specs.
- It is based on ci-services-migrate for now.
- It removes CI events.
- It removes administrator CI projects overview.
- It removes CI application settings.

In 8.4 or 8.5 we can remove redundant tables and columns.


See merge request !1987