feat: Ability to toggle conversation continuity via email (#3817)

Fixes: #3368
Co-authored-by: Pranav Raj S <pranav@chatwoot.com>

db/migrate/20220119051739_enable_conversation_continuity.rb

@@ -0,0 +1,5 @@
+class EnableConversationContinuity < ActiveRecord::Migration[6.1]
+  def change
+    add_column :channel_web_widgets, :continuity_via_email, :boolean, null: false, default: true
+  end
+end