Add initial "devices" view UI

[shane-tomlinson]

The mocks and assets are located at https://github.com/mozilla/fxa/blob/master/features/FxA-16-devices-view/README.md

• Display a list of connected devices returned by the auth server.
  • The current device is the top item in the list.
  • Remaining items are sorted by XXX
  • All items should have a "Last connected" which should be a human readable number formatted like 3 hours ago
• Allow the user to delete a device from the list.
  • When user deletes a device from the list, notify the auth server.
  • When user deletes a device from the list, animate the removal of the item from the list.
  • If the user deletes the current device, notify the browser using a WebChannel message (see #3252)
• Update the UI whenever the user changes the current device name from the browser (see #3252).
• Allow the user to "refresh" the list, which re-fetches the device list from the auth server.

Part of #3250