Update www/cutelyst to latest upstream release.

Release notes are here:
https://dantti.wordpress.com/2019/09/26/cutelyst-2-9-0-and-simple-mail-qt-1-4-0-released/

With highlight:
Cutelyst has got many bug fixes, a few API additions, some
docs fixes, and most importantly it fixed a memory leak introduced
in 2.8.0 that makes applications using chained actions leak.