Add burnup chart FE

What does this MR do?

Add burnup charts behind burnup_charts feature flag - Ref: #6903 (closed)

Feature flag is enabled in !31896 (closed) (also adds changelog)

Screenshots

burnup_events endpoint looks like this:

[
  {
    "created_at":"2020-04-07",
    "action":"add",
    "event_type": "milestone",
    "milestone_id":1,
    "issue_id":441
   }, {
    "created_at":"2020-04-08",
    "action":"remove",
    "event_type": "milestone",
    "milestone_id":44,
    "issue_id":441
  }
]

we turn it into a timeseries array, something like:

[
  { date: "2020-04-06", totalIssues: 1 },
  { date: "2020-04-07", totalIssues: 2 },
  { date: "2020-04-07", totalIssues: 1 }
]

Does this MR meet the acceptance criteria?

Conformity

• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines