chore: Add the ability to use same key for conversation and contact (#3526)

db/migrate/20211208081344_add_attribute_model_index_on_custom_attribute_definition.rb

@@ -0,0 +1,8 @@
+class AddAttributeModelIndexOnCustomAttributeDefinition < ActiveRecord::Migration[6.1]
+  def change
+    remove_index :custom_attribute_definitions, [:attribute_key, :account_id], name: 'attribute_key_index',
+                                                                               if_exists: true
+    add_index :custom_attribute_definitions, [:attribute_key, :attribute_model, :account_id], unique: true, name: 'attribute_key_model_index',
+                                                                                              if_not_exists: true
+  end
+end