Default images do not allow 'alt' attribute

Originally file by @kmcguffey2 at https://github.com/unl/wdntemplates/issues/1464

The default image option for image fields in content types do not have an alternate text field. This causes accessibility issues.

Example: IANR News Article content type - https://ianrnews.unl.edu/admin/structure/types/manage/article/fields/field_image