Consider timezone when computing burndown and burnup charts

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Continuing the discussion in #213334 (comment 372490939)

Now that our burndown and burnup charts are historically-accurate, we lost the feature of user timezone specific charts.

We could implement this in 2 ways:

1. When the user requests the graph data from GraphQL, they include the user's current timezone as one of the arguments.
2. Add a timezone setting under a project / group and then use that when viewing milestones in that project / group.

Another option is to keep things as-is and keep everything aggregated by UTC days.