[D8][UX] Add a toggle in the admin bar for showing/hiding contextual links on mobile

There is no way to use contextual links on mobile devices at the moment, because they are only triggered by hovering your mouse over the editable/configurable items, and mobile devices work with taps.

In D8, they have added an "Edit" button at the far right of the admin bar, that toggles contextual links on/off:

We should have the same.