Images should fit screen width in `columns-1` appearance

[seadowg]

When a select question has the column-1 appearance, the images in the select should fill the "column" (the full screen) width. If an image is not wide enough to fill the screen width, it should not be stretched however (and just be shown full size). This makes it consistent with other columns-n behaviours.

Notes

This was originally discussed on the forum here.

This makes columns-1 inconsistent with the default appearance (which might be expected). We'll want to explicitly document that the default shows images as "thumbnails" to prevent confusion here.