Add "Give Feedback" tab to front page

This allows people who end up at https://input.mozilla.org/ a way to leave feedback. Without this, they'd have to know the url. That was dumb.

Having said that, it's not clear a lot of people go directly to input.mozilla.org to leave feedback. Having said *that*, we have a ton of space in that bar, so might as well add the link for now.

Quick r?