Only use super + left/right for back/forward navigation on Mac

Closes #529.

assets/javascripts/app/shortcuts.coffee

@@ -2,7 +2,7 @@ class app.Shortcuts
   $.extend @prototype, Events
 
   constructor: ->
-    @isWindows = $.isWindows()
+    @isMac = $.isMac()
     @start()
 
   start: ->
@@ -91,14 +91,14 @@ class app.Shortcuts
       when 13
         @trigger 'superEnter'
       when 37
-        unless @isWindows
+        if @isMac
           @trigger 'superLeft'
           false
       when 38
         @trigger 'pageTop'
         false
       when 39
-        unless @isWindows
+        if @isMac
           @trigger 'superRight'
           false
       when 40
@@ -135,14 +135,14 @@ class app.Shortcuts
       when 9
         @trigger 'altRight', event
       when 37
-        if @isWindows
+        unless @isMac
           @trigger 'superLeft'
           false
       when 38
         @trigger 'altUp'
         false
       when 39
-        if @isWindows
+        unless @isMac
           @trigger 'superRight'
           false
       when 40

assets/javascripts/lib/util.coffee

@@ -336,9 +336,6 @@ $.popup = (value) ->
     window.open value.href or value, '_blank'
   return
 
-$.isWindows = ->
-  navigator.platform?.indexOf('Win') >= 0
-
 $.isMac = ->
   navigator.userAgent?.indexOf('Mac') >= 0
 

assets/javascripts/templates/pages/help_tmpl.coffee

@@ -1,5 +1,5 @@
 ctrlKey = if $.isMac() then 'cmd' else 'ctrl'
-navKey = if $.isWindows() then 'alt' else ctrlKey
+navKey = if $.isMac() then 'cmd' else 'alt'
 
 app.templates.helpPage = """
   <nav class="_toc" role="directory">