Lea user can make categories

Pretty straightforward: a logged in user can make categories, which will be used to categorize project types. A pre-requisite for being able to create projects. 

Note: the is_site_admin? method in user.rb is for future use. It just made sense to make both the is_owner? and is_site_admin? methods at once. 
Do you want there to be many categories with the same name or do you want the category names to be unique? If so, there should be a unique constraint when you add the index.