[agent] do not consider soft deleted users in uniqueness checks

Created by: adipasquale

https://trello.com/c/MbRWv7GQ/972-impossible-de-recr%C3%A9er-un-usager-supprim%C3%A9

I fixed 2 different issues preventing re-creating a user that was soft-deleted :

• scope to active users in DuplicateUserFinder => pretty obvious

• bypass users.email unicity => more tricky. The email unicity constraint is at the db level so we cannot just change the scope of the validation (and maybe it's best like that). What I did was that now soft deleted users have a dummy email like user_34@deleted.rdv-solidarites.fr and their original email is backuped in users.email_original. this allows re-creating a new user with the old email.

Also, I couldn't refrain from a little refacto of the user#soft_delete method please indulge me.