Fix visual layout of Unresolved Discussions MR widget on very small screens

What does this MR do?

For #230889 (closed)

...I thought I was going to need more time to investigate the solution to this...

This MR adds a class to the Unresolved Discussions MR Widget.

Notably, it doesn't add the flex-wrap: wrap rule to every MR Widget (via the .mr-widget-body class) because it's likely that wrapping everything would have unexpected and undesirable side effects in one of the many other widgets.

Screenshots

Browser width	Screenshot
Anything above about 450px	As it is today:
At around 425px
Anything below 315px

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