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. 
The update action could have a custom filter for update, `before_action :require_permission, only: [:update]`, and define `require_permission` as a method in this class (probably choose a better name than that). That way, the nested if statements are cleaned up and people will only hit the action if they've got the permission already