Enable UTF grids at zoom 14

Large parcels are somewhat clickable at zoom 14. Addresses #407. I wonder if we should also only enable the UTF zoom limit for surveys with >(arbitrary number) responses? Maybe >1,000?

Zoom 14 looks like this:

![2014-10-29 12 41 55 pm](https://cloud.githubusercontent.com/assets/86435/4830025/be4b6494-5f8a-11e4-83c9-2181863b11be.png)