Use buttons

Note: this won't compile until we release o-ft-buttons 1.8.0.

Nb: this PR is to merge into the `mixins` branch, not master.
Wouldn't it be better to make the first element of a tab focusable, and after clicking a tab, that will be the first element that's focused when clicking tab, but not focusing it by default? I find it slightly confusing, and most people don't use accessibility features