search: add shortcuts to focus

This cribs from Racket's documentation site [1], where pressing "s" will
focus the search bar. This is a modified version of [2], which I choose
to receive and use under the MIT license [3], [4], which is compatible
with MIT-LICENSE.txt.

There's a probably a good way to steer users into this, but "/" is
common thanks to Vim and many websites; "s" is natural (at least to me,
having used Racket's docs so long).

There's probably a "jQuery" way to do this, but I didn't look very hard.

[1]: https://docs.racket-lang.org
[2]: https://github.com/racket/scribble/blob/02ecb223c8e0e4dda3c00739338c96dcd1d5bac6/scribble-lib/scribble/scribble-common.js#L145C2-L152C13
[3]: https://github.com/racket/scribble/blob/02ecb223c8e0e4dda3c00739338c96dcd1d5bac6/LICENSE.txt
[4]: https://github.com/racket/racket/blob/240aa086c548440e0b11641c40cf79e393a560a4/racket/src/LICENSE-MIT.txt

app/assets/javascripts/application.js

@@ -102,6 +102,7 @@ var Search = {
     Search.observeFocus();
     Search.observeTextEntry();
     Search.observeResultsClicks();
+    Search.listenForKeys();
   },
 
   observeFocus: function() {
@@ -149,6 +150,14 @@ var Search = {
     });
   },
 
+  listenForKeys: function() {
+    window.addEventListener("keyup", function(e) {
+      if ((e.key === 's' || e.key === 'S' || e.key === '/') && e.target === document.body) {
+        $('form#search input').focus();
+      }
+    }, false);
+  },
+
   runSearch: function() {
     var term = $('#search-text').val();
     if(term.length < 2) { return false };