New Assignment Builder: allow column stretching

[oscarlevin]

A number of the columns in the new assignment builder exercise search can be too narrow to display all the text (for example, in the Number column, preview activities for active calculus have "Preview Act..." which is not at all helpful). I'm guessing that the widget for the table allows for column stretching, and if so, turning this on would be useful.

[bnmnetp]

It seems column stretching is available in the problem selector but there is no indication it is available unless you move your mouse and pay very close attention.

In the search interface it is not available, but you can move columns. I would like us to disable moving columns and make them resizable instead.