Add Export GeoJSON action for assets

Problem/Motivation

A client would like to export asset(s) geometry as geojson, similar to how the Export KML action works right now. Right now you can get an asset's geojson by manually filtering the /assets/geojson/full/{asset_type} view by name, but this isn't very user friendly.

Any reason not to add an "Export GeoJSON" action? Would it make sense to consolidate these to a single Export geometry action, which then prompts the user to choose the format?

Steps to reproduce

n/a

Proposed resolution

Remaining tasks

User interface changes

API changes

None

Data model changes

None