Fix task lists on closed merge request pages

What does this MR do?

When a merge request is shown, we enable clickable task lists in the description. These were clickable even if the MR was closed. However this was broken in 12.6.

Instead of checking for the existence of a.btn-close, we now check for existence of .description.js-task-list-container, which is only available when the user has permission to edit the merge request.

Related to issue: #196175 (closed)

Screenshots

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

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