Update startup css which fixes login pages

• 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)

What does this MR do?

This MR is the user-facing part of !61635 (closed), which updates the startup css files that haven't been updated in a while 😬

This is also needed for the upcoming top nav and left sidebar refactor.

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

Desc	Before (on master with ?startup_css_only=true)	After (this MR with ?startup_css_only=true)	After (this MR)
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With :combined_menu disabled
With :combined_menu enabled
Dark mode

To just double check we're not making any huge visual regressions, here's a smoke test of a few random pages

MR page	Search

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.