Improve iteration group by label UI

What does this MR do?

In #225500 (closed), the ability to group by labels was added to iteration reports (e.g. /groups/gitlab-org/-/iterations/1/). There are some follow-up UI improvements, mentioned in !51113 (comment 485816203), which this MR implements:

• Adds an alert which lists the labels that don't have associated issues
• No longer shows label grouping tables for labels with no associated issues
• Adds issue's labels to the label grouping table
• Adds a light gray background to the label grouping table
• Fixes scoped label rendering
• Adds page specific CSS to override GlLabel style in GlAlert since I wasn't able to do this using utility classes. I followed the instructions in &3694 for this

Screenshots (strongly suggested)

Old screenshot:

Older screenshot:

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