Emulate pre-2.7 behaviour: when an user is created by the user synchronisation, restore any deleted user with the same username

In FishEye 2.6 and prior, when the user synchronisation could not retrieve an user (either because of a communication error or because the user was removed in LDAP/Crowd/etc.), the user would be deleted.

Later, if an user was re-created with the same username, the original user was restored (along with his admin rights (FE-3712) and all his Crucible data).

Because of this, any transient failures or migration between authentication providers would not be visible to the users. An user would automatically be deleted but then restored in any successive synchronisation from any provider (LDAP, Crowd, etc.), if the username matches.

2.7 changed this behaviour by making the user deletion strict and irreversible. Creating an user with the same username as a deleted user does not restore the old user. However this has caused a lot of headaches for the support team as any migration between the authentication providers are now permanent and does not restore the old users after migration.

A solution for this is to restore the undocumented behaviour of 2.6 and prior versions: restore any user matching a deleted user with the same username.