Roughly support stacked area charts

Summary:
Ref T13279. This adds support for:

• Datasets can have types, like "stacked area".
• Datasets can have multiple functions.
• Charts can store dataset types and datasets with multiple functions.
• Adds a "stacked area" dataset.
• Makes D3 actually draw a stacked area chart.

Lots of rough edges here still, but the result looks slightly more like it's supposed to look.

D3 can do some of this logic itself, like adding up the area stacks on top of one another with d3.stack(). I'm doing it in PHP instead because I think it's a bit easier to debug, and it gives us more options for things like caching or "export to CSV" or "export to API" or rendering a data table under the chart or whatever.

Test Plan:

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: yelirekim

Maniphest Tasks: T13279

Differential Revision: https://secure.phabricator.com/D20498