Embed x, y, z, other data into task polygons

[nbolten]

The tasking manager can provide per-task instructions so long as the data is embedded in the task itself.

For automatically-generated grid tasks, the 'x', 'y', and 'z' attributes of a task are created automatically, but this is not the case for custom GeoJSON task grids (what we use).

We should add 'x', 'y', and 'z' key-value pairs to the 'properties' section of all GeoJSON task features created by this codebase so that they can be referenced in per-task instructions. This will enable:

• Links to street-level imagery services so that mappers don't have to type out the lon/lat/zoom parameters into the URLs manually.
• Links to alternative overhead imagery, vector map sources not embedded / embeddable into the iD editor.

In addition, we have the option to create an "extraProperties" properties object in which to place arbitrary data. We should review whether there are any attributes in our dataset that we would like to include in the instructions. For example, we should be able to determine how many streets feed into an intersection and suggest that there may be this many crossings to map.