Closes #4616894

So far this was only enforced by the MBOXImporter.

Taking information from headers and turning it into our schema's
attributes/relations shouldn't depend on the import script.

To make parsing easier on the hooks side, the 'headers' attribute is
now expected to be an undecoded ascii-only string.

Require cubicweb 3.20 for cubicweb.utils.admincnx.
This means mboximport no longer works with a dbapi connection, so the
ccplugin is broken.

Operations get a cnx instead of a session.

Use int instead of typed_eid.

cubicweb.web.uicfg is now cubicweb.web.views.uicfg, and cw_set replaces
set_relations.

though no class is introduced to read an mbox from stream in remplacement, so
create our own one.

MBOXImporter now have import_mbox(path) in addition to
import_mbox_stream(stream), the former using the std lib mbox class and the
later our own custom class.

This file is generated from doc/email.txt, but for now the generation
process depends on logilab's mkdoc and private xsl files.

This is required since the EmailThread creation logic strongly rely on the order which Email entities are created

The References header may store a space-separated list of values, which was not correctly handled.

The following three main steps are now implemented in three separate
methods:

- database connection: self.connect(appid)
- importer creation: self.create_importer(cnx)
- mbox file import: self.import_mbox_files(importer, filenames)