ProjectsSection: remove unknown projects when (de-)selecting

See T362275#9912455. If a invalid project is entered in the wikitext, the form will now strip it after any changes made to the selected projects. It won't do this normalization unless related changes are made. To fix that, I think data normalization in general should perhaps be moved to the Wish class so that it serves as the first and last line of defense for both storage mediums.

Bug: T362275