Update to Nextcloud 12 using PHP7

* Install PHP7 via a PPA, enable unattended upgrades for the PPA, and switch all of our PHP configuration to the PHP7 install.
* Keep installing PHP5 for ownCloud/Nextcloud packages because we need it to possibly run transitional updates to ownCloud/Nextcloud versions less than 12. But replace PHP5 packages with PHP7 packages elsewhere.
* Update to Nextcloud 12 which requires PHP7, with a transitional upgrade to Nextcloud 11.0.3.
* Disable TLS cert validation by Roundcube when connecting to localhost IMAP and SMTP. Validation became the default in PHP7 but we don't necessarily have a (non-self-)signed certificate and it definitely isn't valid for the IP address 127.0.0.1.

Merges #1140

setup/web.sh

@@ -18,7 +18,11 @@ fi
 # Turn off nginx's default website.
 
 echo "Installing Nginx (web server)..."
-apt_install nginx php5-fpm
+
+apt_install nginx php7.0-cli php7.0-fpm
+
+# Set PHP7 as the default
+update-alternatives --set php /usr/bin/php7.0
 
 rm -f /etc/nginx/sites-enabled/default
 
@@ -40,15 +44,15 @@ tools/editconf.py /etc/nginx/nginx.conf -s \
 	server_names_hash_bucket_size="128;"
 
 # Tell PHP not to expose its version number in the X-Powered-By header.
-tools/editconf.py /etc/php5/fpm/php.ini -c ';' \
+tools/editconf.py /etc/php/7.0/fpm/php.ini -c ';' \
 	expose_php=Off
 
 # Set PHPs default charset to UTF-8, since we use it. See #367.
-tools/editconf.py /etc/php5/fpm/php.ini -c ';' \
+tools/editconf.py /etc/php/7.0/fpm/php.ini -c ';' \
         default_charset="UTF-8"
 
 # Bump up PHP's max_children to support more concurrent connections
-tools/editconf.py /etc/php5/fpm/pool.d/www.conf -c ';' \
+tools/editconf.py /etc/php/7.0/fpm/pool.d/www.conf -c ';' \
 	pm.max_children=8
 
 # Other nginx settings will be configured by the management service
@@ -103,7 +107,7 @@ done #NODOC
 
 # Start services.
 restart_service nginx
-restart_service php5-fpm
+restart_service php7.0-fpm
 
 # Open ports.
 ufw_allow http