Add the "Code" Cycle Analytics section.

1. Record the `wip_flag_first_removed_at` and
   `first_assigned_to_user_other_than_author` metrics for a merge
   request. Use a `merge_request_metrics` table, similar to the one for
   `issues`. Metrics are recorded `after_save`.

2. Move larger queries to a `CycleAnalytics::Queries` module.