Simplify StartupCSS JS Helper and fix autosize problems

What does this MR do?

This MR simplifies the waitForCSSLoaded method and makes it more robust, ie it can be called whenever for added simplicity and reliability.

Diving deeper:

• waitForCSSLoaded:
  • now does just that, wait for the CSS to be loaded. It ensures the code will run only when and if the CSS (of StartupCSS) has already finished loading. It no longer waits for DOMContentLoaded: the reason is to make it work predictably and give the author full control. If they want to run code regardless of DOMContentLoaded, they can use waitForCSSLoaded right away. If not, they do it on DOMContentLoaded.
  • If run AFTER the CSS is loaded, it will execute the action right away. This way the users don't need to do any checks. Always use the same safeguard and regardless of whether it's been already triggered or not, it'll work as expected.
  • if StartupCSS feature flag is off, it will trigger the code right away.

Closes #239387 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team