Logging for prekey fetches, load of log files (#1836)

* Log the files discovered in logPath

I've encountered some logs which include very old entries; and my
suspicion is that we're not cleaning up old log files properly.

* Log prekey fetches (success and failure), just like signed keys

* Force log file information into the final web-ready log

app/logging.js

@@ -93,8 +93,19 @@ function fetch(logPath) {
     return path.join(logPath, file)
   });
 
+  // creating a manual log entry for the final log result
+  var now = new Date();
+  const fileListEntry = {
+    level: 30, // INFO
+    time: now.toJSON(),
+    msg: 'Loaded this list of log files from logPath: ' + files.join(', '),
+  };
+
   return Promise.all(paths.map(fetchLog)).then(function(results) {
     const data = _.flatten(results);
+
+    data.push(fileListEntry);
+
     return _.sortBy(data, 'time');
   });
 }