Load milestone issues tab asynchronously

What does this MR do?

This changes the issues tab under milestones so that it is loaded asynchronously just like the other tabs on this page.

This also fixes a minor bug in the MR tab where we were always showing the project names even for a project milestone.

Related to #197227 (closed)

Screenshots

The page should be the same except that the issues list there is loaded async

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