Roadmap refactoring: move child epics logic from Vuex

What does this MR do and why?

This MR is a part of refactoring roadmaps to use VueApollo effort. Here, we:

• add VueApollo to our application
• move all the logic for child epics (fetching and toggling) from Vuex to epic_item.vue component
• clean up unused methods/parameters

MRs for Roadmaps refactoring

Title	Link
Move non-reactive properties from Vuex to injected	!148520 (merged)
👉 Move fetching child epics logic to epic_item.vue	!148826 (merged)
Move fetching milestones logic	TBD
Move fetching group epics logic	TBD
Move filter params to root component property	TBD
Move local settings to Apollo cache	TBD

MR acceptance checklist

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