[Part 1] Add Vuex store for Roadmap

What does this MR do?

Adds support for Vuex store for Roadmap, laying foundation to migrate Roadmap app to Vuex.

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
• EE specific content should be in the top level /ee folder

#9831 (closed)