Add new basic mentions component

What does this MR do?

This MR adds a new GlMentions component that uses tribute for mentions functionality and is able to grab the list of users to show.

This MR just adds the component without using it—the component will be used in a later MR in the spirit of iteration. This component is intended to eventually replace at.js since it is no longer maintained.

Issue: #207134 (closed)

Screenshots

Screen_Recording_2020-03-05_at_1.32.41_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