webhtml: make top table header sticky (#518)

This keeps the column names visible when scrolling down, which comes in
handy when there are many entries.

internal/driver/webhtml.go

   font-family: 'Roboto Medium', -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol';
 }
 table tr th {
+  position: sticky;
+  top: 0;
   background-color: #ddd;
   text-align: right;
   padding: .3em .5em;