Allow users to be found by @username

What does this MR do?

This allows users to be found by queries that prefix their username with @. It simply strips the @ before querying.

As we reference users across Gitlab by their username presented in the @username format, it makes sense to be able to search them even with the @ prefix.

Personally I've will find this particularly useful when assigning Issues/MRs to other users as I have muscle memory of prefixing their name with @.

Note: the implementation also means that you can search a User's name with @. i.e @user name will match on a user with the name User Name.

Screenshots

Old Behaviour

New Behaviour

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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