Clean up schema for table timelogs

Everyone can contribute. Help move this issue forward while earning points, leveling up and collecting rewards.

• Label this issue
• Close this issue

Per 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 FK: user_id -> users (5 entries violate this rule)
• Missing constraint: CHECK time_spent >= 0