Adjustable web mercator zoom level for opportunity datasets

[ansoncfit]

As of #586, the resolution of isochrones can be adjusted with a zoom parameter in the request. As the discussion there notes, the tricky part will be allowing adjustable zoom levels for the opportunity datasets.

Our initial approach will likely be having users specify a zoom at the region level, and using that as the default when rasterizing opportunity datasets and running analyses. We could check at region creation that the number of pixels is reasonable.

We should also address #566 before enabling.

[abyrd]

We should also address #566 before enabling.

#566 was fixed in #689

[ansoncfit]

We should address this as part of #761. When converting a previously uploaded and validated resource to grid layers, users should specify a desired zoom level.

[trevorgerhardt]

This was completed across a few PRs with #807 being the final one. This was included in the #818 release.