Add default user login page / component

[Decipher]

Is your feature request related to a problem? Please describe.
As a user I want to click on a "login" link provided by the Drupal account menu and have a login experience.

Describe the solution you'd like

• Add default user/login route
• Add DruxtAuthLogin component / page / block

Describe alternatives you've considered

• Add a Route resolver to the Drupal Druxt or Decoupled Router module

Additional context

• https://github.com/druxt/quickstart-druxt-site/blob/develop/nuxt/pages/user/login.vue