Refactoring the code for pub sub (#155)

- We were using the attribute name 'channel' to store pubsub tokens, which was confusing.
- switched to faker from ffaker
- spec for contact.rb

db/migrate/20191016211109_rename_channel_attribute_name_in_models.rb

@@ -0,0 +1,9 @@
+class RenameChannelAttributeNameInModels < ActiveRecord::Migration[6.1]
+  def change
+    rename_column :users, :channel, :pubsub_token
+    rename_column :contacts, :chat_channel, :pubsub_token
+
+    add_index :users, :pubsub_token, unique: true 
+    add_index :contacts, :pubsub_token, unique: true 
+  end
+end