Improve tribute @mentions UX

What does this MR do?

This MR improves tribute @mentions UX.

Previously, the member's display name and username showed on the same line. If this member was a group with long names and usernames, then this made the autocomplete menu very wide.

This MR shows the display name and username on separate lines, and for group members only the actual group name and its parent are shown rather than the complete path. Additionally, group members are searched using their group display name only and not any other text in their path (i.e. any of their ancestor groups).

This change is part of a feature which is behind the tribute_autocomplete feature flag, and is only active in issues (not MRs or epics).

Issue: #236003 (closed)

Screenshots

Screen_Recording_2020-09-22_at_7.07.13_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