DEV: Refactor draft attributes for CategoryList and TopicList.

Avoid repeating the same logic in a bunch of places which will allow us
to make changes to the draft attributes easier in the future.

app/models/draftable_list.rb

@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+class DraftableList
+  include ActiveModel::Serialization
+
+  def initialize(user)
+    @current_user = user
+  end
+
+  def draft_key
+    @draft_key || Draft::NEW_TOPIC
+  end
+
+  def draft_sequence
+    @draft_sequence || DraftSequence.current(@current_user, draft_key)
+  end
+
+  def draft
+    @draft || Draft.get(@current_user, draft_key, draft_sequence) if @current_user
+  end
+
+  def draft_key=(key)
+    @draft_key = key
+  end
+
+  def draft_sequence=(sequence)
+    @draft_sequence = sequence
+  end
+
+  def draft=(draft)
+    @draft = draft
+  end
+end