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.

- The account menu now has a "devices" tab. This tab contains a table
  with device names (currently a short hash) and device ids (a long
  hash).

  In the "devices" tab, there is an "add device" button which allows
  for exporting the current account to the Ring, giving a pin to the
  user.

- When the client encounters old-format accounts, it will trigger a
  migration popup which asks the user for a password. This password will
  be used to create an archive and encrypt it. One password will be
  asked for each Ring account to migrate.

Change-Id: I2d579aa270ed1edbfdd7e5f361c6e6078d3fdef1
Tuleap: #896