Add parent-child work item table

What does this MR do and why?

Related to #362125 (closed)

Adds parent-child work item table and model with associations. This new table is needed for implementing work items hierarchy planned in &7448

Migration

bin/rails db:migrate:up:main VERSION=20220511144946

== 20220511144946 AddWorkItemParentChildTable: migrating ======================
-- create_table(:work_item_parent_links, {})
   -> 0.0348s
== 20220511144946 AddWorkItemParentChildTable: migrated (0.0349s) =============

bin/rails db:migrate:down:main VERSION=20220511144946

== 20220511144946 AddWorkItemParentChildTable: reverting ======================
-- drop_table(:work_item_parent_links)
   -> 0.0332s
== 20220511144946 AddWorkItemParentChildTable: reverted (0.0333s) =============

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.