Add color_mode_id column to users table

What does this MR do and why?

Related to !141459 (merged). Adds color_mode_id column to users table. Adds background database migration to migrate dark color mode id: 2 where users have selected dark mode theme id: 11.

The execution plan for the UPDATE query(it updates no records though): https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/25846/commands/81504

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