Fix horizontal scroll issue on mobile

Fixes purescript#154
Solution taken from <https://stackoverflow.com/a/36380425>

Author: andys8

Diff for: index.html

@@ -16,7 +16,12 @@
   <meta name="viewport" content="width=device-width, initial-scale=1">
 
   <style>
+    html, body {
+      overflow-x: hidden;
+    }
+
     body {
+      position: relative;
       background-color: white;
       font-family: 'Roboto';
       color: rgb(77, 77, 77);