Feature/add inbox specs (#192)

* Add base Inbox association and validation specs

* `#facebook?``

* `#add_member` and `#remove_member`

* cleanup

* Rubocop

* Rubocop but again

really this time

app/models/inbox.rb

@@ -4,6 +4,8 @@ class Inbox < ApplicationRecord
   validates :account_id, presence: true
 
   belongs_to :account
+
+  # TODO: should add associations for the channel types
   belongs_to :channel, polymorphic: true, dependent: :destroy
 
   has_many :contact_inboxes, dependent: :destroy