Split API messages from ULS

It's not a lot of messages, but this is a very common and basic extension,
and it's often one of the first to be translated, so it's good to make its
translation easier.

To be committed together with the corresponding
translatewiki configuration change:
I33ca817b457abc5504917745c2730ec223e263e0

Change-Id: I13ea351553319583051778cfb6c55f843b5f86e5

extension.json

@@ -36,7 +36,8 @@
 	},
 	"MessagesDirs": {
 		"UniversalLanguageSelector": [
-			"i18n"
+			"i18n",
+			"i18n/api"
 		]
 	},
 	"AutoloadClasses": {