fix(l10n): localize report start and end datetimes

The "during" start and end datetimes of "Download Report" (/stats/report)
were using server-rendered times, i.e.: strftime("%Y-%m-%d %H:%M:%S").
When server and client time zones differed, the times of the report
would not match the client time zone.
These datetimes are now localized similarly to the rest of the report.

locust/templates/report.html

@@ -44,7 +44,7 @@ <h1>Locust Test Report</h1>
             {% if show_download_link %}
                 <p class="download"><a href="?download=1">Download the Report</a></p>
             {% endif %}
-            <p>During: <span>{{ start_time }} - {{ end_time }}</span></p>
+            <p>During: <span class="l10n datetime">{{ start_time }}</span> - <span class="l10n datetime">{{ end_time }}</span></p>
             <p>Target Host: <span>{{ host }}</span></p>
         </div>
 
@@ -219,6 +219,20 @@ <h2>Charts</h2>
                 ]
             });
         }
+
+        $(".l10n.datetime").html((index, currentContent) => {
+            if (!currentContent || !currentContent.includes(" ") || !currentContent.includes(":")) {
+                return currentContent;
+            }
+
+            let [date, time] = currentContent.split(" ");
+            let utcDateTime = new Date();
+            let [year, month, day] = date.split("-")
+            utcDateTime.setUTCFullYear(year, parseInt(month) - 1, day);
+            utcDateTime.setUTCHours(...(time.split(":")));
+
+            return utcDateTime.toLocaleString();
+        });
     </script>
 </body>
 </html>