Implement goal charting

[neilrqm]

Neighbourhoods have activity goals specified, and we want to optionally include these goals on the neighbourhood chart. the API already includes a goal value in its stats response, but it is always set to 0. The goal is a scalar value, and we want to draw a horizontal line at that value. Ideally this line would be the same hue as the neighbourhood's line colour, but a lighter (or darker?) tone. In any case, there are two steps here. First, the API needs to pull goals from the neighbourhood data and set the goal value in its responses. Second, the web app needs to add the goal as a line in the chart, perhaps adjusting the goal line's colour while retaining the hue.

Alternately, it may be easier for the API to generate an object that can be charted like it does for the statistical data, rather than just sending a scalar value.