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/comments_controller_test.rb

@@ -188,7 +188,7 @@ class CommentsControllerTest < ActionDispatch::IntegrationTest
     context "create action" do
       should "create a comment" do
         assert_difference("Comment.count", 1) do
-          post_auth comments_path, @user, params: {comment: FactoryBot.attributes_for(:comment, post_id: @post.id)}
+          post_auth comments_path, @user, params: { comment: { post_id: @post.id, body: "blah" } }
         end
         comment = Comment.last
         assert_redirected_to post_path(comment.post)
@@ -196,7 +196,7 @@ class CommentsControllerTest < ActionDispatch::IntegrationTest
 
       should "not allow commenting on nonexistent posts" do
         assert_difference("Comment.count", 0) do
-          post_auth comments_path, @user, params: {comment: FactoryBot.attributes_for(:comment, post_id: -1)}
+          post_auth comments_path, @user, params: { comment: { post_id: -1, body: "blah" } }
         end
         assert_redirected_to comments_path
       end