PersonModel : implementation of BannedContactModel

- add a new class BannedContactModel. This model loads at start of the
client a list from the daemon to figure which ContactMethods are
banned.

- it updates the list (for now, we can only ban) when a peer is banned.

[Guillaume: fix typo in ci msg]

Change-Id: I2367c8918dd4f743ebbce2b0c1c90e5fde71c45b