[FIX] web: list: execute group button with keyboard navigation

Recent work on the list view allowed to add buttons in group
headers (see 015958dc). However, we couldn't execute the action
of the button by pressing Enter, because the Enter keydown was
stopPropagated by another handler (folding the group).

This rev. makes this work, and also improves a bit the style of
the buttons in group headers.

Task 1915702