add trashable concern on user

spec/factories/users.rb

@@ -4,6 +4,7 @@
 #
 #  id                     :bigint           not null, primary key
 #  avatar_url             :string
+#  deleted_at             :datetime
 #  email                  :string           default(""), not null
 #  encrypted_password     :string           default(""), not null
 #  name                   :string           not null

spec/models/user_spec.rb

@@ -4,6 +4,7 @@
 #
 #  id                     :bigint           not null, primary key
 #  avatar_url             :string
+#  deleted_at             :datetime
 #  email                  :string           default(""), not null
 #  encrypted_password     :string           default(""), not null
 #  name                   :string           not null