[Part 2] Fetch epics for Roadmap via Vuex

What does this MR do?

Adds support fetching epics in Roadmap via Vuex actions and mutations.

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides (https://about.gitlab.com/handbook/engineering/quality/guidelines/test-engineering/).
• EE specific content should be in the top level /ee folder

Closes #9831 (closed)