Set instance name

Nextcloud has a concept of an "instance name" which is shown to the user in various places, including as the name part of the FROM of email notifications. We should probably set this to a simple but sensible default at installation time, perhaps $domain Nextcloud or $domain Stackspin Nextcloud. It can easily be changed later by the user in the Nextcloud UI so we should make sure that we don't override it again on upgrade.