add basic user revert functionality

app/logical/user_revert.rb

@@ -0,0 +1,28 @@
+# reverts all changes made by a user
+class UserRevert
+  THRESHOLD = 1_000
+  class TooManyChangesError < RuntimeError ; end
+
+  attr_reader :user_id
+
+  def initialize(user_id)
+    @user_id = user_id
+  end
+
+  def process
+    validate!
+    revert_post_changes
+  end
+
+  def validate!
+    if PostVersion.where(updater_id: user_id).count > THRESHOLD
+      raise TooManyChangesError.new("This user has too many changes to be reverted")
+    end
+  end
+
+  def revert_post_changes
+    PostVersion.where(updater_id: user_id).find_each do |x|
+      x.undo!
+    end
+  end
+end