[BE] Create system-defined hard-coded work item types in application code

The work_item_types table was re-organized to have a constant set of IDs.

Unless there are reasons not to, we should convert further to use a hard-coded list of work item types.

Also convert the related tables:

• work_item_types
• work_item_hierarchy_restrictions Sharding Key Investigation for `work_item_hiera... (#553477 - closed)
• work_item_related_link_restrictions Sharding Key Investigation for `work_item_relat... (#553478 - closed)
• work_item_widget_definitions

See https://docs.gitlab.com/development/cells/#static-data for guidance