Add jpg's as webp fallback for Safari

Replaced webp backgrounds with jpg's because couldn't figure out
how to use Modernizr (detect webp support for CSS fallback) for Hugo.

Also added back testimonial section with leader testimonials.

assets/sass/custom.scss

@@ -77,3 +77,10 @@ p, h3, h4 {
   // width: auto;
   // height: auto;
 }
+
+#counter {
+  background-position: center;
+  background-repeat: no-repeat;
+  // background-size: auto;
+  // background-size: contain;
+}