Group issues by label within the iteration report

What does this MR do?

When viewing an iteration report (e.g. at /groups/gitlab-org/-/iterations/1), all the issues that are part of that iteration are listed.

This MR adds the ability to also group issues by label within the iteration report.

Issue: #225500 (closed)

Frontend reviewer info

I have split this MR into two commits—commit 1 refactors code and commit 2 is the main one to review which is the feature implementation.

commit 1:

• Moves code from iteration_report_tabs.vue to a new component iteration_report_issues.vue without changing its logic. The new component renders a list of issues, and we reuse this component in commit 2 to 1) show a list of all issues, and 2) show a list of issues grouped by the selected filter
• Also moves test code to a new file iteration_report_issues_spec.js
• Adds the following new code so that the new component works properly:
  • In iteration_report_issues.vue
    • New code in the apollo smart query object to emit the count of the received issues result({ data }) { this.$emit('issueCount', data[this.namespaceType]?.issues?.count); }
  • In iteration_report_tabs.vue
    • Event handler for the new IterationReportIssues component @issueCount="handleIssueCount"
    • Which invokes method handleIssueCount(count) { this.issueCount = count; }
    • Which stores the emitted count in data property issueCount: undefined

commit 2:

• This is the main commit to review, which contains the feature implementation for #225500 (closed)

Screenshots (strongly suggested)

Screen_Recording_2021-01-12_at_5.27.19_pm

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

Related to #225500 (closed)