Search across full name for mentions autocomplete

What does this MR do?

Right now, you cannot search across the full name mentions autocomplete due to the space between the first and second name. This MR updates the search so that typing @johndoe will match name John Doe, and the name is matched first.

This is for #36705 (closed) which was initially implemented for tributejs, the autocomplete library we're migrating to, but since it might not be rolled out soon due to some issues we're also adding a fix for our current implementation here which uses atjs

Screenshots (strongly suggested)

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

Related to #36705 (closed)