Implement Shortcuts in Ember

Closes #2988, #2752
Ref #1463, #2984,
 # Shortcuts via Keymaster
- 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'`