Convert add-on submission time to seconds (nvaccess#17193)

Part of nvaccess#16681, fixup of PR 17091

Summary of the issue:
Add-on submissionTime in json metadata files is in milliseconds, but it needs to be converted to seconds to be transformed in a publicationDate

Description of user facing changes
None

Description of development approach
Convert add-on submissionTime to seconds in addonStore/models/addon.py

source/addonStore/models/addon.py

@@ -188,7 +188,8 @@ def isPendingInstall(self) -> bool:
 	def publicationDate(self) -> str | None:
 		if self.submissionTime is None:
 			return None
-		return datetime.strftime(datetime.fromtimestamp(self.submissionTime), "%x")
+		# Convert `self.submissionTime` to seconds.
+		return datetime.strftime(datetime.fromtimestamp(self.submissionTime // 1000), "%x")
 
 
 class _AddonManifestModel(_AddonGUIModel):