Validate Nextcloud 18.0.0 on release "2020-01-17"

https://github.com/nextcloud/server/wiki/Maintenance-and-Release-Schedule
https://github.com/nextcloud/server/milestone/77
https://download.nextcloud.com/server/releases/

Refs: #2021 #2092 #2108 #2109 #2111 #2112 #2113 #2114 #2115 #2117 #2119