Add urldates to bibliography entries with url

bibliography.bib

@@ -37,6 +37,7 @@
  year = 2013,
  pages = "150--153",
  url = "https://magazin.spiegel.de/EpubDelivery/spiegel/pdf/91346615",
+ urldate = "2021-04-22",
 }
 @inbook{nara-report,
  title = "Goering, Albert",
@@ -47,6 +48,7 @@
  location = "College Park",
  keywords = "Quelle",
  url = "https://catalog.archives.gov/id/57339937",
+ urldate = "2021-04-29",
 }
 @inbook{nara-verhör,
  title = "Goering, Albert",
@@ -57,4 +59,5 @@
  location = "College Park",
  keywords = "Quelle",
  url = "https://catalog.archives.gov/id/57321728",
+ urldate = "2021-04-29",
 }