update bootstrap 4.3.1, update gulpfile, npm start script added, change SCSS file names for proper compiling

vendor/bootstrap/scss/_tooltip.scss

@@ -7,7 +7,7 @@
   // Our parent element can be arbitrary since tooltips are by default inserted as a sibling of their target element.
   // So reset our font and text properties to avoid inheriting weird values.
   @include reset-text();
-  font-size: $tooltip-font-size;
+  @include font-size($tooltip-font-size);
   // Allow breaking very long words so they don't overflow the tooltip's bounds
   word-wrap: break-word;
   opacity: 0;