Reduce space bar shortcut timeout

assets/javascripts/app/shortcuts.coffee

@@ -47,7 +47,7 @@ class app.Shortcuts
       when 27
         @trigger 'escape'
       when 32
-        if not @lastKeypress or @lastKeypress < Date.now() - 1000
+        if not @lastKeypress or @lastKeypress < Date.now() - 500
           @trigger 'pageDown'
           false
       when 33