cubicweb #3381521 improve notification view to not have wildly separate code paths [validation pending]

currently the notification view gets a list of recipients that are either CWUser entities or (email, lang) tuples. This leads to separate code paths to handle the two cases which has caused a number of bugs (especially as this is hardly tested).

prioritynormal
typeenhancement
done in3.19.0
load2.000
load left0.000
closed by#7e35d62c67a9 [notification] use an InternalManager object for notifications if we have an (email, lang) tuple
patch[req] if a user has a None property, fall back to the default [applied][server] Allow setting an InternalManager's preferred language [applied][notification] use an InternalManager object for notifications if we have an (email, lang) tuple [applied]