[notification] merge BaseNotificationView and NotificationView They are in the same module now.

[notification] move notification view in ``sobject.notification`` It has no user outside this module. This enforce serversideness of notification and allow future cleanup. No backward compat is set up to prevent circular import. The class has no other user anyway. (closes 2845144)

remove unused import pyflakes say they are unused. Tests suite confirms.

one more merge

[notification] introduce an official `notify_on_commit` function It provides a proper notification api instead of replacing it by yet another clumsy one. Closes #2837251

[cubicweb/doc] Replace dc_type() by cw_etype dc_type() is may be translated. Leading to terrible terrible damage.

[dataimport] backout 6947201033be (related to #2788402) (and add a try: except to cache the intended error) The problem actually comes from the ``MassiveObjectStore`` in the ``dataio`` cube, so it should be corrected there. Here, we only protect it with a ``RuntimeWarning`` so that the user can see the problem. ``value`` is set to ``None`` (whence to ``NULL`` from a database standpoint), so that the data can be nevertheless inserted in the database. However, only the keys present in ``row`` are actually non-'``NULL``'. The real solution is to work out the issue in ``MassiveObjectStore`` directly. The current try/except should only be a temporary hack.

merge default heads