Support backing up shared data directory

Somewhat recently the wekan chart added a new persistent volume called "shared data volume". This contains avatars and may contain attachments as well depending on configuration and/or admin actions. It should be backed up. However, the wekan chart is not prepared with the right knobs to do that in the usual stackspin way:

• No existingClaim value to create the pvc outside of the chart in our wekan kustomization.
• [ ] No podAnnotations to set the backup.velero.io/backup-volumes annotation. (Done earlier.)