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

db/schema.rb

@@ -274,6 +274,7 @@ ActiveRecord::Schema.define(version: 2022_01_21_055444) do
     t.boolean "pre_chat_form_enabled", default: false
     t.jsonb "pre_chat_form_options", default: {}
     t.boolean "hmac_mandatory", default: false
+    t.boolean "continuity_via_email", default: true, null: false
     t.index ["hmac_token"], name: "index_channel_web_widgets_on_hmac_token", unique: true
     t.index ["website_token"], name: "index_channel_web_widgets_on_website_token", unique: true
   end