Make tribute mentions autocomplete avatar and text bigger

What does this MR do?

We are in the process of migrating from atjs (deprecated and jQuery-based) to tribute for autocomplete under the feature flag tribute_autocomplete (defaulted to off).

When the tribute_autocomplete is turned on, the avatar and font is a bit small. This MR increases their size.

Related to &4002

Screenshots (strongly suggested)

before	after

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