[books] Allow timezone changes when importing from KOReader

Turns out you need a city-based timezone for DST stuff to work properly.
The US/Eastern timezone doesn't mess with DST because it can be so wonky
in different regions. So while we fix timezone defaulting to a
DST-friendly timezone too.

vrobbler/apps/scrobbles/admin.py

@@ -139,6 +139,7 @@ class ScrobbleAdmin(admin.ModelAdmin):
         "media_type",
         "long_play_complete",
         "source",
+        "timezone",
     )
     ordering = ("-timestamp",)