cubicweb #2413437 crash in ldapuser2ldapfeed script [validation pending]

extids are plain strings, and we combine them with unicode data: explosion ensues

(also the script goes a bit too fast wrt. user confirmation)

prioritynormal
typebug
done in3.15.2
load0.100
load left0.000
closed by#f52bb4226020 [ldapuser2ldapfeed] fix confusing script structure and decode the extid to avoid an UnicodeDecodeError (closes #2413437)