Improve pdf viewer: do not show child works in admin dashboard by default

[jillpe]

Summary

Pals would like child works to not show in the admin dashboard by default to reduce clutter. However, they would like the option to view them in case their users want access to them. They would like this to be either a feature flipper or an additional tab alongside "All Works" and "Your Works" (whichever is the easier lift).

Screenshots

### Additional tab location 

Feature Flipper location

Acceptance Criteria

• As a user, when I visit my works on the admin dashboard, by default, I do not see child works
• As a user, when I visit my works on the admin dashboard, I have the option to see child works