Add Inky-based, WIDM-styled email templates (#212)

Adds twig/inky-extra and twig/cssinliner-extra to build the password
reset and email confirmation emails from a shared responsive layout,
styled after the quiz's green/black look, with Dutch copy in the
show's dramatic tone.

Dockerfile

@@ -32,6 +32,7 @@ RUN set -eux; \
 		opcache \
 		zip \
 		gd \
+		xsl \
 	;
 
 # https://getcomposer.org/doc/03-cli.md#composer-allow-superuser