This patch implements multi-device support

- The account creation wizzard has now two options "Existing Ring
  account" and "New Ring account".

  "Existing Ring account": Allows for fetching a Ring account archive
  from the DHT. Requires pin and password.

  "New Ring account": This is the previously existing wizard. It was
  modified to ask for a password which will be used to encrypt the
  account archive. This password is then required for exporting the
  account on the Ring.

- Creating a new Ring account with the "+" button now triggers the
  account creation wizard.

Change-Id: I6840d968e52de014ab0aa9dd6f5bc1ab31f9e9ec
Tuleap: #958