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. 
I decided to leave this as-is, since it's so short.