#1722 Split Barnes & Noble External IDs into two types

[Ahasuerus]

As per https://isfdb.org/wiki/index.php/ISFDB:Moderator_noticeboard#Barnes_.26_Noble_External_IDs:

• Remove B&N 978/979 values from the External ID field and move them to the ISBN field
• Create a new "BN EAN" External ID type
• Move all 294 BN EANs to the new External ID Type, leaving all other BN IDs associated with the current External ID Type
• Update [[Template:PublicationFields:ExternalIDs]]
• Create two cleanup reports. One will look for BN EANs that do not start with "294". The other one will look for non-EAN BN IDs that start with 294, 978 or 979