Add mutual links page

locales/ja/common.json

@@ -4,5 +4,6 @@
     "tos": "yude.jp サービス利用規約",
     "yes_playing": "{{playing}} をプレイ中",
     "listening": "{{listening}} を再生中",
-    "close": "閉じる"
+    "close": "閉じる",
+    "mutual": "相互リンク"
 }