Explicitly call gitlab-ui config function

What does this MR do?

This MR explicitly imports and execute the gitlab UI config function in the main.js file.

This is to allow the proper configuration to be set both in development and production environments.

The missing proper configuration on the production environment generates a series of bugs, this issue, for example, lists one: #222515 (closed).

I've verified that this fix applies the correct config on the production build and removes the bug of the issue listed above.

note: an earlier attempt to fix this was done in the gitlab-ui codebase: gitlab-ui!1505 (closed) unsuccessfully

Also, this MR lead to this MR to update the docs: !35016 (merged)

Screenshots

No visual changes directly

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