api: refactor api attribute declarations.

Replace the `method_attributes` and `hidden_attributes` methods with
`api_attributes`. `api_attributes` can be used as a class macro:

    # include only the given attributes.
    api_attributes :id, :created_at, :creator_name, ...

    # include all default attributes plus the `creator_name` method.
    api_attributes including: [:creator_name]

or as an instance method:

    def api_attributes
       [:id, :created_at, :creator_name, ...]
    end

By default, all attributes are included except for IP addresses and
tsvector columns.

test/functional/users_controller_test.rb

@@ -84,11 +84,10 @@ class UsersControllerTest < ActionDispatch::IntegrationTest
       end
 
       should "render the current users's profile in json" do
-        get_auth profile_path(format: :json), @user
+        get_auth profile_path, @user, as: :json
         assert_response :success
 
-        json = as(@user) { @user.as_json(methods: @user.full_attributes + @user.method_attributes) }
-        assert_equal(json, response.parsed_body)
+        assert_equal(@user.upload_limit, response.parsed_body["upload_limit"])
       end
 
       should "redirect anonymous users to the sign in page" do