FEATURE: editing_grace_period_max_diff to force revisions in grace period

If a user performs a substantive edit of 20 chars or more during grace period
we will store a revision to track the change

This allows for better auditing of changes that happen during the grace period

spec/models/post_spec.rb

@@ -674,7 +674,7 @@ describe Post do
       context 'second poster posts again quickly' do
 
         it 'is a ninja edit, because the second poster posted again quickly' do
-          SiteSetting.expects(:editing_grace_period).returns(1.minute.to_i)
+          SiteSetting.editing_grace_period = 1.minute.to_i
           post.revise(changed_by, { raw: 'yet another updated body' }, revised_at: post.updated_at + 10.seconds)
           post.reload