NEWS: add short overview section

I decided to err on the side of brevity, since the details are just
below.
NEWS: deprecate notmuch deliver

notmuch-deliver has no commits for about 2.5 years. notmuch-insert has
all the features that deliver does, and as far as I understand the
error handling has now caught up.
version: bump to 0.19
debian: update debian/NEWS and debian/changelog

Plagiarize my upstream summary for debian, and mention the new
upgrade.
Merge tag '0.19'

notmuch 0.19 release