Provide instructions to merge LDAP users

[strk]

Some users had registered multiple account, lacking a password reset form. Now they ask to delete the duplicated once, as we finally (via mantra/IRC) get to talk with them. Still, there's no coded procedure to delete the accounts.

The simplest operation would be just deleting the account, but would be a lossy operation in that we'd loose any information about the account shall it ever be needed for historical reason (only needed if the user ever left any trace in the services).

Another option would be "blocking" the account while adding a motivation explaining what the new account is (see #1668 for blocking accounts).

Finally, there might be some "redirection" support in LDAP, but I'm not aware of that.