Add authentication via devise

Signed-off-by: Tobias Eidelpes <tobias@eidelpes.info>

test/models/user_test.rb

@@ -0,0 +1,7 @@
+require 'test_helper'
+
+class UserTest < ActiveSupport::TestCase
+  # test "the truth" do
+  #   assert true
+  # end
+end