Draft: Add an icon to issues on subepics when filtering by Epic on Vue

What does this MR do?

Related to #213002 (closed)

On !27212 (merged) there were some changes that needed to be replicated on issuables_list/components/issuable.vue.

This basically determines if an issue belongs to a subepic when filtering by epic, and shows an indicator icon with a tooltip that says: "This issue is in a child epic of the filtered epic".

Screenshots

List with sub-epic issues included

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