Use binary (1024-based) prefixes instead of decimal (1000-based) for
storage unit conversion. Previously dividing by 1000000 and 1000,
now correctly dividing by 1048576 (1024*1024) and 1024.
Fixes#2502
...when the tooltip extends beyond the page.
I've also added the tooltips (for both storage and activity) logic to a js file `intro_tooltips.js` and imported it into `intro.tpl`