Create/apply taxonomy for Features and Technologies

[grtjn]

1. Take current values for Features and Tech facet, and clean up both lists. Features are about functionality, technology about libraries and products. Viz Widgets is a technology (you can download this from DMC), not a feature. Showing geospatial map is a feature, Google Maps however a technology.
2. Check completeness of the lists, and ideally add descriptions for the items, for showing in the UI.
3. Put the cleaned list in the features/technologies files at https://github.com/marklogic/demo-cat/tree/master/data, and use these to constrain input in the create page. Don't allow custom additions anymore.
4. Migrate existing values from existing demos to match these new lists. Could perhaps be done with a migrate script.