[UX] Turn the installer into a 1-minute installer

The Drupal installer takes quite long and frankly, its only because we accumulated a lot of information we need to display. Backdrop can diverge from this and truly provide a 1-2 screen experience.

For example: 1) Default to English, and provide the option to change it - instead of making it a "step" in the process. 2) Put Database configuration and admin username/password/e-mail on one page. 3) Skip all the stuff around Site name, site e-mail address (just use admin e-mail), default country, timezone, update notifications. If people want to change that, finding it through the admin UI is much more valuable to build up understanding.