Resolve "Add support for parsing work item references at group level"

What does this MR do and why?

Allow referencing group level work items by URL or references. This currently only works by using references in notes and work item descriptions that exist at the project level. Enabling this for group level work items in a follow-up MR to keep this one smaller

Follow up doesn't require a lot of changes, but with specs it might be considerably larger.

Steps to test

1. Enable the namespace_level_work_items FF in rails console Feature.enable(:namespace_level_work_items)
2. Create a new epic in http://localhost:3000/groups/flightjs/-/work_items
3. Reference the new epic using a full url or full reference (example flightjs#1) in another issue that exists in a project. For group level work items, this will only work in an issue's comment using the preview button instead of posting the comment
4. Short reference should also work in another epic in the same group #1(using preview)

MR acceptance checklist

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Related to #432555 (closed)