Add ability to expand epics in roadmap

What does this MR do?

Enables the user to expand epics in the roadmap view if it contains sub-epics.

This MR is currently blocked by !18957 (merged)

Feature flags: :roadmap_graphql for turning on GraphQL and :unfiltered_epic_aggregates for turning on weights in GraphQL

Related issue: #7077 (closed)

Screenshots

Old video but demonstrates same functionality: expand-epics-video

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

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