Implement Shortcuts in Ember

Closes #2988, #2752
Ref #1463, #2984,

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Shortcuts via Keymaster
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- Added KeyMaster to bower dependencies. KeyMaster is a minimal keyboard  shortcuts library.
- Added `ShortcutsRouteMixin` for routes that will use shortcuts.
  Currently, only routes can have shortcuts. See the extensive comment  at the top of core/client/mixins/shortcuts-route.js` for a  description of how to implement shortcuts.

 ## Other Changes
- Injected popover service into ApplicationRoute
- Created `EditorRouteBase` mixin for the `editor.new` and  `editor.edit` routes to mixin.
- `StyleBodyMixin` now calls `this._super()` on `activate` and  `deactivate` to play nicely with other mixins.

 ## Shortcuts and Stubs implemented
 #### Application-Wide
- `'esc':'closePopups'` shortcut **stub** to close popovers, modals, and notifcations

 #### Editor Shortcuts
- `'ctrl+s, command+s': 'save'` note that `command` is the  `meta` key.
- `'ctrl+alt+p': 'publish'`
- `'ctrl+alt+z': 'toggleZenMode'`
There is a minor problem with this, in that `Ctrl+s` will work on mac as well as `⌘+s`. It's a pretty small niggle - @JohnONolan or @PaulAdamDavis might be able to give some feedback on how common/uncommon it is for both `Ctrl` and `⌘` to work on a mac?

I'd like to fix it, but can see from keymaster there's no smart handling of this yet so the solution might be a bit ugly.

Still it's not a massive priority IMO - I think it's more important to start getting more of the shortcuts in place.