Preview attribute table information on record show page for vector data

[thatbudakguy]

Note: this ticket applies only to Vector datasets (shapefiles, points, polygons, etc.). For the version specific to Raster data, see #1172.

This ticket is for exploring ways to add a preview of attribute table information so that users can get a better sense of what data is contained in the attributes. This is different from inspection ("what info is at the pixel that I clicked?") and is instead intended to represent a general survey or broad understanding of all the data or a preselected sample, to help the user make an informed decision about downloading. It's more accessible and can be displayed without asking the user to pick a point using a mouse.

The goal is to show an example of what features the data contains, as in the mockup:

We could try parsing the ISO 19110 XML linked from the dct_references_s field (if there is one) to get extra information about the naming of the attributes and what they mean.

As noted by the GIS team, some kinds of data (in particular census data) may have polygons that have null attributes associated, so we might need to be a little careful about just picking a feature at random (or the first feature) to display a sample. There might also be another way to give a good summary of what's "in" the data besides just showing the first feature.