Implement offline POST method for starting an offline participation

spec/factories/offlines.rb

@@ -4,6 +4,7 @@
 #
 #  id         :integer          not null, primary key
 #  end_time   :datetime
+#  name       :string           not null
 #  start_time :datetime         not null
 #  created_at :datetime         not null
 #  updated_at :datetime         not null

spec/models/offline_spec.rb

@@ -4,6 +4,7 @@
 #
 #  id         :integer          not null, primary key
 #  end_time   :datetime
+#  name       :string           not null
 #  start_time :datetime         not null
 #  created_at :datetime         not null
 #  updated_at :datetime         not null