[2020] Add new tab is HIGHLY questionable. Please remove that button. We already have too few space up there.

This button was added a long time ago (it feels like five years?) but it looks like it was only shown if you had less than 10 stash tabs. A change in this refactor has made it unconditional, so we will revert that back to the old behaviour.

Edit: Upon some digging, the old behaviour was it would vanish as soon as there wasn't room for it (which took 10 tabs if they were the default size). This is what we'll revert back to.