Current page to manage project categories doesn't allow you to associate projects to a category.

In other words, it means that if you want to add 10 projects to a specific category, you have to visit all 10 projects and associate each one of them to the category.

After a design discussion with dhudson, we considered it would be important to add a project picker to both create and edit project category, allowing a quick administration of the associated categories.

This board picker should offer an experience similar to the fields in the "Create Issue" dialog, where you start typing, select an option and move to the next selection (check attached .gif).