define default display for `@key`
Created by: GVogeler
@key can be assigned to cei:persName, cei:placeName, cei:orgName, cei:bibl.
It can contain either a local key or an external.
Local keys refer to tei:person/@xml:id structure in /db/mom-data/metadata.person.public/*.tei.xml for cei:persName and to the geo-coordinates structure in development for #299.
External keys are URIs (e.g. viaf, gnd, geonames.org identifier, zotero). They should start with https?:// and could therefore be used as hyperlinks.