Add WebSerial ESPTool link to Absolute Newest section

Add a link to the Adafruit WebSerial ESPTool on board download pages
for boards with bootloader_info.installer == true (ESP32 variants).
This gives users a way to flash .bin files downloaded from S3.

Addresses #1572

Author: makermelissa-piclaw

_includes/download/board.html

@@ -220,6 +220,9 @@ <h3>Absolute Newest</h3>
   </p>
   <div>
     <a class="download-button-unrecommended" href="https://adafruit-circuit-python.s3.amazonaws.com/index.html?prefix=bin/{{ board_id }}/">BROWSE S3<i class="fas fa-arrow-circle-right" aria-hidden="true"></i></a>
+    {% if bootloader_info and bootloader_info.installer and bootloader_info.installer == true %}
+    <a class="download-button-unrecommended" href="https://adafruit.github.io/Adafruit_WebSerial_ESPTool/">OPEN WEBSERIAL ESPTOOL<i class="fas fa-arrow-circle-right" aria-hidden="true"></i></a>
+    {% endif %}
     <div class="clear"></div>
   </div>
 </div>