Support "placeholder" dashboard blocks

Problem/Motivation

Drupal CMS would find it incredibly useful if we could add blocks to the dashboard that may, or may not, actually exist.

Right now, if you have a block in your dashboard and that block plugin is unavailable, you'll get the "Broken or missing block" block when you try to render it, along with a warning in your logs. That's standard core behavior.

What would be really neat is if it were possible to mark blocks in a dashboard as being "optional", so that if the block doesn't actually exist at render time, it's just silently removed from the layout, with no warning. Layout section components already have an "additional" property where you can store additional flags, so maybe we could check for the component's existence (by asking BlockManager) and check if the component has a dashboard_optional flag in its additional values.