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/unit/comment_test.rb

@@ -72,7 +72,7 @@ class CommentTest < ActiveSupport::TestCase
 
           dmail = Dmail.last
           assert_equal(<<-EOS.strip_heredoc, dmail.body)
-            @#{CurrentUser.name} mentioned you in a \"comment\":/posts/#{@comment.post_id}#comment-#{@comment.id} on post ##{@comment.post_id}:
+            @#{@comment.creator.name} mentioned you in a \"comment\":/posts/#{@comment.post_id}#comment-#{@comment.id} on post ##{@comment.post_id}:
 
             [quote]
             Hey @#{@user2.name} check this out!
@@ -195,7 +195,7 @@ class CommentTest < ActiveSupport::TestCase
       should "not allow upvotes by the creator" do
         user = FactoryBot.create(:user)
         post = FactoryBot.create(:post)
-        c1 = FactoryBot.create(:comment, :post => post)
+        c1 = create(:comment, post: post, creator: CurrentUser.user)
 
         exception = assert_raises(ActiveRecord::RecordInvalid) { c1.vote!("up") }
         assert_equal("Validation failed: You cannot upvote your own comments", exception.message)