* 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