Use token number in disputes

[Catrya]

To add extra security to users when they open a dispute and prevent malicious users from writing to them pretending to be admin, it would be good if mostro gave each user a token number when opening a dispute (a different number for the buyer and the seller) and when an admin takes the dispute, both tokens are shown to the admin, who must tell the users the number when writing to him

Here how is implemented it on @lnp2pbot

[Catrya]

Duplicated in #337