Updated generated CSS with CSS 2020-08-24

What does this MR do?

• Updates the generated CSS with newly crawled critical CSS for signin page + dark theme, those were created automatically
• This fixes the flash of unstyled content for dark theme, sign in page
• Reactivates Querystring parameters to test easier startup.css for the different setups (?startup_css=true)
• Adds crossorigin attributes also to link tag as unmatching don't work (learning the hard way, tested with proxyman that this should now work)

Everything from startup.css is still hidden behind a feature flag

This is not fixing yet the different nav bar colors and collapsed sidebar problem, we will tackle this in another MR before activating fully

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