Provide period_field query option for Insights
At https://gitlab.com/gitlab-org/quality/team-tasks/issues/129#note_172920967 we'll want to specify that we sort and group by closed_at rather than created_at.
At https://gitlab.com/gitlab-org/quality/team-tasks/issues/129#note_172920967 we'll want to specify that we sort and group by closed_at rather than created_at.