feat: Ability to lock to single conversation (#5881)

Adds the ability to lock conversation to a single thread for Whatsapp and Sms Inboxes when using outbound messages.

demo: https://www.loom.com/share/c9e1e563c8914837a4139dfdd2503fef

fixes: #4975

Co-authored-by: Nithin David <1277421+nithindavid@users.noreply.github.com>

db/migrate/20221109065043_add_lock_conversation_to_single_thread.rb

@@ -0,0 +1,5 @@
+class AddLockConversationToSingleThread < ActiveRecord::Migration[6.1]
+  def change
+    add_column :inboxes, :lock_to_single_conversation, :boolean, null: false, default: false
+  end
+end