Starting with 2.4, Z-Push no longer provides tarballs on their download server. The only options are getting the code from their git repository or using one of their distribution packages. Their Ubuntu 18.04 packaes don't seem to actually work in Ubuntu 18.04, so thinking ahead that's currently a bad choice. In 78d1c9be6e we switched from doing a git clone to using wget on their downloads server because of a problem with something related to stash.z-hub.io's SSL certificate. But wget also seems to work on their source code repository, so we can use that.
git clone (which uses curl) underneath was failing. Curiously, the same
git clone command would work on my macos host machine.
From the screenshot it looks like curl was somehow not able to negotiate
the connection. Might have been a missing CA certificate for Comodo, but
I was not able to determine if that was the issue.
fixes#1393closes#1387closes#1400
* 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
