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/bulk_update_request_test.rb

@@ -37,7 +37,7 @@ class BulkUpdateRequestTest < ActiveSupport::TestCase
 
     context "#update_notice" do
       setup do
-        @forum_topic = FactoryBot.create(:forum_topic)
+        @forum_topic = create(:forum_topic, creator: @admin)
       end
 
       should "update the cache" do
@@ -70,7 +70,7 @@ class BulkUpdateRequestTest < ActiveSupport::TestCase
           mass update aaa -> bbb
         '
 
-        @bur = FactoryBot.create(:bulk_update_request, :script => @script)
+        @bur = create(:bulk_update_request, script: @script, user: @admin)
         @bur.approve!(@admin)
 
         assert_enqueued_jobs(3)
@@ -111,7 +111,7 @@ class BulkUpdateRequestTest < ActiveSupport::TestCase
 
     context "that has an invalid alias" do
       setup do
-        @alias1 = FactoryBot.create(:tag_alias)
+        @alias1 = create(:tag_alias, creator: @admin)
         @req = FactoryBot.build(:bulk_update_request, :script => "create alias bbb -> aaa")
       end
 
@@ -172,8 +172,8 @@ class BulkUpdateRequestTest < ActiveSupport::TestCase
 
     context "with an associated forum topic" do
       setup do
-        @topic = FactoryBot.create(:forum_topic, :title => "[bulk] hoge")
-        @post = FactoryBot.create(:forum_post, :topic_id => @topic.id)
+        @topic = create(:forum_topic, title: "[bulk] hoge", creator: @admin)
+        @post = create(:forum_post, topic: @topic, creator: @admin)
         @req = FactoryBot.create(:bulk_update_request, :script => "create alias AAA -> BBB", :forum_topic_id => @topic.id, :forum_post_id => @post.id, :title => "[bulk] hoge")
       end