FIX: saving drafts unconditionally increases sequence

Previously we only changed sequence on ownership change, this
cause a race condition between tabs where user could type for a
long time without being warned of an out of date draft.

This change is a radical change and we should watch closely.

Code was already in place to track sequence on the client so no
changes are needed there.

app/models/draft_sequence.rb

@@ -48,7 +48,7 @@ end
 #  id        :integer          not null, primary key
 #  user_id   :integer          not null
 #  draft_key :string           not null
-#  sequence  :integer          not null
+#  sequence  :bigint           not null
 #
 # Indexes
 #