emacs: use def instead of initial-input for notmuch-show-browse-urls
This is the non-deprecated way to use completing-read. Additionally
the old use was broken when using ivy for completing-read. For user's
using completing-read-default they won't see the default URL now, but
if they hit enter it will be visited. Alternatively they can select
it with M-n.
From the completing-read documentation for initial-input:
This feature is deprecated--it is best to pass nil for INITIAL-INPUT
and supply the default value DEF instead. The user can yank the
default value into the minibuffer easily using M-n.
Additionally collection is now all urls, rather than all but the
first. I'm not sure why "(cdr urls)" was previously done.