added higher z-index to featured image to make sit on top of sidebar …

It will now trim the columns to the number defined in the map.

Data Layer updates a "last edit date" field on the Item

Conflicts:
	django/website/data_layer/tests/item_tests.py

Data layer techdebt

Conflicts:
	django/website/hid/tests/categorize_items_tests.py

Barchart time period

When a term is related to two items and is removed from one of them,
the other item should not show as being modified.

In this case for listing terms

Allow the chart count widget to have a time period. When present, apply the time period and display a legend detailing what the time period is.

In this case for checking taxonomies exist

In this case for counting taxonomies

Allow widgets to display error messages in the front end by raising a WidgetError (while other exceptions only display a generic message so as to avoid potential security leak)