Support GeoJSON uploads for creating opportunity datasets.

[trevorgerhardt]

Closely mirrors the way Shapefiles are converted into grids.

Refs #747

[trevorgerhardt]

Based on a discussion last year, we might want to follow the approach in #761.

This doesn't prevent or hinder following the approach in #761 whenever that is implemented.

If we're not going to switch to that workflow yet, it might be good at this stage to reduce some of the repetition here, by refactoring some of the fromShapefile code into reusable methods.

Ok, I'll take a look and see if there is something to pull out there 👍