Optimize header.js with code splitting

What does this MR do?

This optimizes header.js to reduce the overall size of main.js (saves around 17KB) :

• Converts the Status Modal Trigger to HAML + Vanilla JS
• Loads the Status Modal only on Click of the menu button
• Moves the dropdown click tracking into an idleCallback to reduce the render blocking effects

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

Closes #246525 (closed)