Project level project selection for WI children

What does this MR do and why?

Following !150711 (merged), this MR adds the ability to select a project to associate new child items with when adding them via the hierarchy (child items) widget.

!150711 (merged) added the ability to do this for namespace-level items (epic work items), and this MR adds the ability to do this for project-level items (issues, etc.).

This MR also adds a note to the docs about project selection when creating a new task or objective or key result.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Visit or create an issue

2. Add tasks to it as children. Check you can select any project within the same group or sub-group.

3. Visit or create an objective

4. Add child objectives or key results to it. Check you can select any project within the same group or sub-group.

Related to #436255 (closed)