Add startup-css-check in CI

• 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 startup-css-check CI job which verifies that the generated files in app/assets/stylesheets/startup/ are up-to-date.

Why are there so many file changes in this MR!?

It's been a long time since we've updated Startup CSS and it used to use a semi-manual process. This MR establishes the new baseline using the new Startup CSS generation.

Screenshots (strongly suggested)

How to test?

You can either:

1. Simply open a page locally and check for any significant style changes while the application.css loads.
2. Try applying the following patch and visiting a page with ?startup_css_only=true to prevent the application.css from loading. You can then compare this page with one that does not have the ?startup_css_only parameter and compare for differences.

0001-Add-startup_css_only-condition.patch

Description	Before (master with ?startup_css_only=true)	After (this MR with ?startup_css_only=true)	After (this MR with full load)
Sign in
Register
Project page + signed out + Feature.enable(:combined_menu)
Project page
Project page + sidebar collapse
Project page + Feature.enable(:sidebar_refactor)
Project page + Feature.enable(:combined_menu)
Dark mode

You'll notice that updating the startup CSS in this MR fixes the following experiences on initial page load:

• Initial colors when dark mode
• Sidebar refactor
• Identicon on sidebar
• Register page

There's some things which we're choosing not to fix in this MR, namely the startup CSS for when a GitLab instance has a system header/footer and when the performance bar is open.

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.