On-demand loading of select2 CSS

What does this MR do?

The target of this MR is to reduce the overall CSS size significantly as all CSS for select2 is only loaded when we actually have select2 elements on screen.

• Added all select2 relevant CSS in a Lazy bundle already in a previous MR
• Now the select2 css is removed from the main css bundle in this MR
• The Lazy CSS Bundle for select2 is loaded through a JS function
• All the JS changes are simply the wrapping of the loading function
• Spec changes are around adding additional waits for the CSS being loaded

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