Fix infinite vertical stretch in Description field of dialog from nuxeo-add-to-collection-documents-button

Steps to reproduce:

1. Create a Document that can be added to a Collection
2. Click Add to Collection button from Document actions
3. In Collections dropdown, type name of a new Collection ('+' sign should appear next to entry) and click
4. In Description text-area, paste a very long value

Expected: Text area stretches to a maximum height such that the "Cancel" and "Add" buttons remain accessible on screen.

Actual: Text area vertically grows indefinitely; "Cancel" and "Add" are inaccessible until either changing browser zoom or clicking off the dialog and reopening.