Add speed tower test, tweak start gcode preheating

#258
Hot end temperature target is now set to 50 degrees less than target temperature for homing and probing, and then set to the actual target temperature before the print starts.

js/loadscripts.js

@@ -18,6 +18,7 @@ var scripts = /*html*/ `
 <script defer src="js/retraction.js"></script>
 <script defer src="js/temperature.js"></script>
 <script defer src="js/acceleration.js"></script>
+<script defer src="js/speed.js"></script>
 <script defer src="js/minimumlength.js"></script>
 `;
 document.write(scripts);