Offer migration from special fields in keywords to special fields as fields

[koppor]

Refs #6621

[calixtus]

Devcall decision: Agenda for 5.3:

• Set default sync to keywords to disabled No sync of specialfields to keywords as default #6846
• Understand problems with synchronisation
• Create migration for keywords

[koppor]

@DominikVoigt I have some detailed information in this. Should be a "Fingerübung", because of your experience at #7334