make the 🔗 button copy branch links to clipboard

to accomplish this, I generalised emoji tooltips to a shared Tooltip class.
in the long run I'd like to transform all existing `title=""` tooltips into these for stylistic consistency with the rest of the website, but this is good enough for now.

I also ended up cleaning up some old code from before the /b rework.

template/components/_head.hbs

@@ -45,6 +45,7 @@ clever to do while browser vendors figure that out, we'll just have to do a cach
     import "treehouse/tree.js";
     import "treehouse/emoji.js";
     import "treehouse/command-line.js";
+    import "treehouse/overlay.js";
 </script>
 
 <meta property="og:site_name" content="{{ config.user.title }}">