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 struggled with that - I think it's possible to have a renewables category and an efficiency category for the same thing, like installing new solar panels to replace older inefficient ones would fall under efficiency, but installing new ones to replace grid power would fall under renewables. Now I'm thinking there should be a uniqueness constraint on the combination of category type and category name, which would allow that flexibility, while preventing repetition within a type. Thanks for getting me thinking about it!