Milestone description: add autocomplete [RUN AS-IF-FOSS]

What does this MR do?

There's autocomplete for Markdown fields in different places (e.g. issue description, MR description, comments, ...). However, this feature is currently not available for milestone description.

So this MR enables autocomplete for milestone description. This applies when adding and editing milestones of projects or groups.

🛠 with ❤ at Siemens

/cc @bufferoverflow

Screenshots

Milestone_description_auto_complete

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a 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