Adds functionality for receiving emails and storing them in the db.

This isn't being used anywhere (see #4005). We stopped having it tested because there are multiple problems. Last usage was in lino_book.projects.noi1e.

We use a fork of the original django-mailbox package because we inject fields into the models, which is possible only for real Lino models. (I tried to add this field injecting magic also to plain Django models, by calling lino.core.model.Model.django2lino() already when the model is prepared (see lino.core.inject), but that's not trivial and causes other issues.

The original django-mailbox causes the demo fixture to fail with "eml: this field cannot be blank".

(This module's source code is available here.)


Plugin(site, app_label, app_name, ...)