Add Global Keyboard Shortcuts

Not all of these have been fully implemented yet.

**Jump To**
* `g` then `h` - Home (Latest)
* `g` then `l` - Latest
* `g` then `n` - New
* `g` then `u` - Unread
* `g` then `f` - Favorited
* `g` then `c` - Categories List

**Navigation**
* `u` - Back to topic list
* `k` / `j` - Newer/Older conversation or post
* `o` or `Enter` - Open selected conversation
* <code>`</code> - Go to next section
* `~` - Go to previous section

**Application**
* `c` - Create a new topic
* `n` - Open notifications menu
* `/` - Search
* `?` - Open keyboard shortcut help

**Actions**
* `f` - Favorite topic
* `s` - Share topic
* `<Shift>` + `s` - Share selected post
* `r` - Reply to topic
* `<Shift>` + `r` - Reply to selected post
* `l` - Like selected post
* `!` - Flag selected post
* `b` - Bookmark selected post
* `e` - Edit selected post
* `d` - Delete selected post
* `m` then `m` - Mark topic as muted
* `m` then `r` - Mark topic as regular
* `m` then `t` - Mark topic as tracking
* `m` then `w` - Mark topic as watching

app/assets/javascripts/discourse/controllers/keyboard_shortcuts_help_controller.js

@@ -0,0 +1,12 @@
+/**
+  This controller is used to display the Keyboard Shortcuts Help Modal
+
+  @class KeyboardShortcutsHelpController
+  @extends Discourse.Controller
+  @namespace Discourse
+  @uses Discourse.ModalFunctionality
+  @module Discourse
+**/
+Discourse.KeyboardShortcutsHelpController = Discourse.Controller.extend(Discourse.ModalFunctionality, {
+  needs: ['modal']
+});