Add autofocus to label search fields

What does this MR do?

Adds autofocus: true to the shared label nav search field.

Purely trivial, a simple UX improvement. When navigating to project/group issues, the search field will be autofocused to search existing issues.

Screenshots

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

(ref: mdn)

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers