feat: Contactable Inboxes API (#2101)

- Add endpoint which lists inboxes through which a contact can be contacted
- Conversation creation API auto-creates contact_inbox for specific channels [ Twilio, email, api]
- Ability to send the initial message payload along with the conversation creation
- Fixes #1678 ( issue saving additional attributes for conversation )

app/models/contact.rb

@@ -31,7 +31,9 @@ class Contact < ApplicationRecord
   validates :account_id, presence: true
   validates :email, allow_blank: true, uniqueness: { scope: [:account_id], case_sensitive: false }
   validates :identifier, allow_blank: true, uniqueness: { scope: [:account_id] }
-  validates :phone_number, allow_blank: true, uniqueness: { scope: [:account_id] }
+  validates :phone_number,
+            allow_blank: true, uniqueness: { scope: [:account_id] },
+            format: { with: /\+[1-9]\d{1,14}\z/, message: 'should be in e164 format' }
 
   belongs_to :account
   has_many :conversations, dependent: :destroy