Clean up schema for table historical_data
From https://gitlab.com/gitlab-com/infrastructure/issues/1709 (private to GitLab employees):
- Wrong data type: created_at should be timestamptz
- Wrong data type: updated_at should be timestamptz
- Missing constraint: created_at NOT NULL
- Missing constraint: updated_at NOT NULL
- Missing constraint: active_user_count NOT NULL
- Missing constraint: date UNIQUE (1 entry violates this rule- 2016-07-15)
What is this table even used for, and do we need it in the first place?