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/forum_post_test.rb

@@ -53,7 +53,7 @@ class ForumPostTest < ActiveSupport::TestCase
       context "outside a quote block" do
         setup do
           @user2 = FactoryBot.create(:user)
-          @post = FactoryBot.build(:forum_post, :topic_id => @topic.id, :body => "Hey @#{@user2.name} check this out!")
+          @post = build(:forum_post, creator: @user, topic: @topic, body: "Hey @#{@user2.name} check this out!")
         end
 
         should "create a dmail" do
@@ -63,7 +63,7 @@ class ForumPostTest < ActiveSupport::TestCase
 
           dmail = Dmail.last
           assert_equal(<<-EOS.strip_heredoc, dmail.body)
-            @#{CurrentUser.name} mentioned you in topic ##{@topic.id} (\"#{@topic.title}\":[/forum_topics/#{@topic.id}?page=1]):
+            @#{@user.name} mentioned you in topic ##{@topic.id} (\"#{@topic.title}\":[/forum_topics/#{@topic.id}?page=1]):
 
             [quote]
             Hey @#{@user2.name} check this out!
@@ -169,7 +169,7 @@ class ForumPostTest < ActiveSupport::TestCase
     end
 
     should "initialize its creator" do
-      post = FactoryBot.create(:forum_post, :topic_id => @topic.id)
+      post = create(:forum_post, topic: @topic, creator: @user)
       assert_equal(@user.id, post.creator_id)
     end