Move slot content from RelatedIssuesList to RelatedIssuableItem

What does this MR do?

This MR is for the related issues block on the Issues page. RelatedIssuesList renders a list of RelatedIssuableItems:

RelatedIssuesList	RelatedIssuableItem

RelatedIssuableItem is used in three places:

• related_issues_list.vue
• related_merge_requests.vue
• blocking_merge_request_body.vue

The component has two slots, dueDate and weight, but only related_issues_list.vue passes down slot content. I believe this was done to prevent the two latter merge request components from trying to display the due date and weight, but merge requests inherently don't have a due date or weight, and they won't have these properties in their data:

As a result, there's no real reason to use slots here, so I moved the slot code directly into RelatedIssuableItem.

Related issues with related MR

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