users: refactor password reset flow.

The old password reset flow:

* User requests a password reset.
* Danbooru generates a password reset nonce.
* Danbooru emails user a password reset confirmation link.
* User follows link to password reset confirmation page.
* The link contains a nonce authenticating the user.
* User confirms password reset.
* Danbooru resets user's password to a random string.
* Danbooru emails user their new password in plaintext.

The new password reset flow:

* User requests a password reset.
* Danbooru emails user a password reset link.
* User follows link to password edit page.
* The link contains a signed_user_id param authenticating the user.
* User changes their own password.

test/functional/password_resets_controller_test.rb

@@ -0,0 +1,25 @@
+require 'test_helper'
+
+class PasswordResetsControllerTest < ActionDispatch::IntegrationTest
+  context "The passwords resets controller" do
+    setup do
+      @user = create(:user)
+    end
+
+    context "show action" do
+      should "work" do
+        get password_reset_path
+        assert_response :success
+      end
+    end
+
+    context "create action" do
+      should "work" do
+        post password_reset_path, params: { user: { name: @user.name } }
+
+        assert_redirected_to new_session_path
+        assert_enqueued_email_with UserMailer, :password_reset, args: [@user]
+      end
+    end
+  end
+end