invitations: reload invitations when registering account

On iOS, when the app is in the background, invitations are received
and saved by the notifications extension. When Jami becomes active in
the foreground, those invitations may not be visible. This
patch reload invitations from the file when registering an account.

jami-client-ios#250
Change-Id: I9ae3d8ec285c94081c5a6d1ceeb31f9058673dcf