Timetable Editor: Add trip_short_name field

[landonreed]

This field already exists in the database for trips, we just need a place to edit it in the timetable editor. Long term, we should probably make this a part of the trip pattern, but that doesn't currently exist for trip_headsign either, so an incremental fix is just to update the editor to include this field.

Starting point for this would be to add an entry for tripShortname below tripHeadsign here:

datatools-ui/lib/editor/selectors/timetable.js

Lines 46 to 52 in b3f79af

	{
	name: 'Trip Headsign',
	width: 200,
	key: 'tripHeadsign',
	type: 'TEXT',
	placeholder: 'Destination via Transfer Center'
	}