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