Support prefixed user IDs for nextcloud

We are attempting to switch from the Nextcloud app social_login to its fork oidc_login (nextcloud#1077 (closed)). Unfortunately they behave differently in the user ID they feed to Nextcloud: social_login makes that stackspin-$ID and oidc_login just $ID, where $ID is the id as specified by the sub attribute (by default).

Because we don't want to attempt a migration at this point to actually change the user IDs, we need to add an extra attribute to the OIDC id token from the dashboard, containing the stackspin- prefixed ID, which we can tell Nextcloud/oidc_login to use as the user's ID.