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long time without being warned of an out of date draft.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.

db/migrate/20200512064023_change_draft_sequence_to_bigint.rb

@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+class ChangeDraftSequenceToBigint < ActiveRecord::Migration[6.0]
+  def change
+    change_column :drafts, :sequence, :bigint, default: 0, null: false
+    change_column :draft_sequences, :sequence, :bigint, null: false
+  end
+end