Add categories to pull screen - 1

[kirtangajjar]

Description of the Change

Add categories to the pull screen. This is a part 1 of 2 of this issue. This PR only lists categories from posts.

Closes #428

How to test the Change

1. Open a pull screen and select any site
2. Ensure that you see "Categories" column on it.
3. Ensure that the categories link to their original site.
4. Ensure step 1-3 for both internal and external connections.

Changelog Entry

Added - categories to pull screen

Credits

Props @kirtangajjar

Checklist:

• [x] I agree to follow this project's Code of Conduct.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my change.
• [x] All new and existing tests pass.