voxygen: avoid unnecessary allocation

Cow::to_string() goes through &str even for an Owned variant,
Cow::into_owned() passes Owned variants through as-is. As it happens,
the values here are always Owned (see comment in try_msg_ctx()
implementation).

voxygen/i18n-helpers/src/lib.rs

@@ -228,7 +228,7 @@ pub fn localize_chat_message(
                     })
                 },
             }
-            .to_string()
+            .into_owned()
         },
     };