add position 'counter' to MessagePart
sort message parts by position
Contact.list_by_group classmethod to retrieve all distincts contacts indexed by all found groups (or unknow one)
Contact.groups list column
list contacts by group Api
a BaseMessage class, MdaMessage inherit
date insert for User model
get subject from message, no need to link mail anymore
Revert "a BaseMessage class, MdaMessage inherit"

This reverts commit 1eb4ed909a9575b5a0a074d091064b5beb7c9b8f.
no more threadlookup cql structure to setup