Feature flags MVC

See merge request gitlab-org/gitlab-ee!7433

Simplify runner registration token resetting

See merge request gitlab-org/gitlab-ee!7511

CE upstream - 2018-10-03 09:21 UTC

See merge request gitlab-org/gitlab-ee!7781

The Unleash API is mounted in:
/api/v4/feature_flags/unleash/:project_id/

- Make name unique
- Make description to be as much as 255 characters longs
- Make the list paginated and ordered