models: remove belongs_to_creator macro.

The belongs_to_creator macro was used to initialize the creator_id field
to the CurrentUser. This made tests complicated because it meant you had
to create and set the current user every time you wanted to create an
object, when lead to the current user being set over and over again. It
also meant you had to constantly be aware of what the CurrentUser was in
many different contexts, which was often confusing. Setting creators
explicitly simplifies everything greatly.

test/functional/post_flags_controller_test.rb

@@ -3,9 +3,7 @@ require 'test_helper'
 class PostFlagsControllerTest < ActionDispatch::IntegrationTest
   context "The post flags controller" do
     setup do
-      travel_to(2.weeks.ago) do
-        @user = create(:user)
-      end
+      @user = create(:user, created_at: 2.weeks.ago)
     end
 
     context "new action" do
@@ -17,10 +15,7 @@ class PostFlagsControllerTest < ActionDispatch::IntegrationTest
 
     context "index action" do
       setup do
-        @user.as_current do
-          @post = create(:post)
-          @post_flag = create(:post_flag, :post => @post)
-        end
+        @post_flag = create(:post_flag, creator: @user)
       end
 
       should "render" do