Bug 991287 - Update links on webmaker.org/mentor page

https://bugzilla.mozilla.org/show_bug.cgi?id=991287
So @k88hudson got me thinking about how this appears for our translators, and it occurs to me that as long as we have `<meta charset="utf-8" />`, we don't need entity references for smart quotes after all…? Not necessarily a nit for this PR, but something I have on my mind for my future changes. I know you're the expert on this though, would love to hear your thoughts. (I'm pretty adamant about using proper typographic quotes though, so I'll stick with entities if the characters themselves don't work x-broswer)