Cleanup compatibility shims/po[nl]yfills, etc. after we drop legacy browser support

[gabrielmontagne]

This issue is to link and keep track of the little (or medium) changes we've had to add to the grid to be able to run it on legacy browsers. Even though the general policy is to leave the shimming at the app level, we've needed to leak some of that to the grid-level code.

At some point in the future 🌴 🌞 , we'll remove this and close this issue.

Let's inaugurate with #29

[gabrielmontagne]

Just to note than, we even have a horrible isIE module in our code -- that will be nice to get rid off when the time comes.