Move identical methods across project and group soft deletion to a single concern/module.

Problem to solve

The project delayed-delete and group delayed-delete features are identical in nature. Some of their methods, model scopes, associations, etc are also identical (or nearly so), and could be moved to a single concern/module where they could be reused for both features.

Proposal

• Migrate the projects.marked_for_deletion_at column to a new column in a new table: project_deletion_schedules.marked_for_deletion_on (assuming a date column is as good as a datetime column)
• Combine the methods related to these 2 features into a model concern (if possible).