Use outline instead of border for edit mode style
This works around what appears to be an obscure Firefox bug: after
dragging a form into any input when editing a french-adjective, the
third form’s input becomes blank, its text and cursor invisible.
Highlighting drop targets using outline instead of border makes the
issue go away; it’s slightly less pretty because now the original border
of the input is still there, but it doesn’t look too bad, in my opinion.
Fixes #74 .