[reverted] Improve at.js members autocomplete matching

What does this MR do?

This MR improves the @ autocomplete matching due to some users' issues with the top results not being as expected, such as in #36705 (comment 507434394).

When searching for a user, e.g. @win, users whose name OR username starts with win are now shown as the top result in the autocomplete menu.

This MR also increases the results list from 5 to 10 for members autocomplete due to other suggestions that 5 results is not enough, such as in #199080 (closed).

Screenshots (strongly suggested)

Screen_Recording_2021-02-12_at_1.05.27_pm

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 #118597