Allow setting feature flags per GitLab group

What

Allows setting feature gates per GitLab group through the API. Allows POST /api/v4/features/feature_name?value=true&group=some/group/path to act like Feature.enable(:feature_name, Group.find_by_full_path('some/group/path')).

This builds on support for setting feature flags by project, as implemented in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24184.

Not to be confused with setting features by Flipper feature_groups, which are for batch updating pre-registered collections.

Why

This will be useful for ChatOps via gitlab-com/chatops#50 (comment 139313062)

Acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug