Fix previewing quick actions for epics

What does this MR do?

It fixes epic notes and they now support previewing quick actions for epics. Following steps needed to be done:

• added args to preview_markdown path for groups
• changed epic.id to epic.iid for the path (same as for other types)
• added Epic as a supported type for quick actions preview
• added feature specs covering basic quick action for epic

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• 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

Issue #8199 (closed)