Create a group_by problem for the reduce section

[BrooklinJazz]

Group reminders by user id.

[%{user_id: 1}, %{user_id: 1}, %{user_id: 2}] -> %{1 => [%{user_id: 1}, %{user_id: 1}], 2 => [%{user_id: 2}]}