Add mutual links page

locales/en/common.json

@@ -4,5 +4,6 @@
     "tos": "yude.jp Terms of Service",
     "yes_playing": "Playing {{playing}}",
     "listening": "Listening to {{listening}}",
-    "close": "Close"
+    "close": "Close",
+    "mutual": "Mutual links"
 }

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"
 }