Add generate:startup_css yarn script

• Feedback issue
• Full context MR (permanent draft)
• Update startup css which fixes login pages
• Add generate:startup_css yarn script
  • Add startup-css-check in CI (depends on parent MR)
• Fix startup-css-check as-if-foss rules
• Add startup_css Dangerfile

What does this MR do?

This MR adds the scripts for the new yarn generate:startup_css. This will be used to regenerate the files in app/assets/stylesheets/startup and will also have a CI check.

Screenshots

It works 🎉

Happy path	If unexpected error

It also generates the most up-to-date Startup CSS. In this MR !62486 (merged), you can see how this will fix some issues. Unfortunately, that MR had to be reverted because of an MR which would have changed the Startup CSS !62556 (merged) was merged right before hand.

In hindsight, we should have merged the scripts and CI check first, which is what we're going to do now.