Show blocking epics in the roadmap
### Problem to solve
> As a User of Roadmaps,
> I need to see dependencies displayed on the Roadmap
> So I can ensure the sequence of work is being executed properly
### Intended users
* [Parker (Product Manager)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#parker-product-manager)
* [Delaney (Development Team Lead)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#delaney-development-team-lead)
### Proposal
For an MVC, we'd mimic the behavior available in issue boards:
* Display a `blocked` icon to the left of the epic title
* Display a popover with details of the blocking item on hover/click of `blocked` icon
Showing issues in the roadmap and arrows between objects can be follow on iterations.
### Competitive Analysis
Behavior for GitLab issue boards

issue