[notification] add operation responsible for sending email notification when an entity is updated

backport stable branch

don't accept None to avoid error

[selectors] add accept_none to EClassSelector so we can specify to not ignore None from outer join in some cases

can't edit in_state anymore, hide it

update test, record a failing test (commented out)

merge

[form] important refactoring of inlined forms to get proper separation of form object creation / rendering

__linkto may only appears in root form

[form] put unknown constructor args into extra_kwargs and consider them when looking for field display values

update to new file schema

forgotten that one

[form] replace is_subform by parent_form, carrying more information at the same cost

move low-level form handling to base form class

fix warning