cubicweb #5190001 mail sending should not block commit() [validation pending]

the notification view was (accidentally?) switched from using a threaded task (SendMailOp) to synchronously sending mails over smtp, which makes everything terrible.

priorityimportant
typebug
done in3.20.6
load0.500
load left0.000
closed by#df34de44f40f [notification] don't block while sending mails
patch[notification] don't block while sending mails [applied]