emacs: use def instead of initial-input for notmuch-show-browse-urls

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.