Support user- and group-specific feature flags
!21687 (merged) added feature flag checks to the existing
feature_available? methods we already use.
We need to take this further and support user-specific (this feature is enabled for
rspeicher) and group-specific (this feature is enabled for members of
gitlab-com) checks so that we can enable a feature flag in more fine-grained ways.
Related chat thread: https://gitlab.slack.com/archives/C84B5KFLJ/p1539265764000100