users: refactor change password page.

* Fix users being redirected back to the change password page after
  successfully changing their password.
* Move passwords controller out of /maintenance/ namespace.
* Add tests.

test/functional/passwords_controller_test.rb

@@ -0,0 +1,26 @@
+require 'test_helper'
+
+class PasswordsControllerTest < ActionDispatch::IntegrationTest
+  context "The passwords controller" do
+    setup do
+      @user = create(:user, password: "12345")
+    end
+
+    context "edit action" do
+      should "work" do
+        get_auth edit_user_password_path(@user), @user
+        assert_response :success
+      end
+    end
+
+    context "update action" do
+      should "work" do
+        put_auth user_password_path(@user), @user, params: { user: { old_password: "12345", password: "abcde", password_confirmation: "abcde" } }
+
+        assert_redirected_to user_path(@user)
+        assert_equal(nil, User.authenticate(@user.name, "12345"))
+        assert_equal(@user, User.authenticate(@user.name, "abcde"))
+      end
+    end
+  end
+end