Before, sending was done synchronously on the api/send POST call. Now
we just build the MIME message and mark the message for sending, and
queue the actual SMTP (or internal) sending for the worker. We use
this opportunity to add fine-grained delivery statuses for each
recipient, something that traditional email systems are unable to do!
Expanding on that, this (too large) PR adds an initial Elasticsearch
index of threads. It also overhauls the build and CI system, using
Poetry instead of Pip.
Currently we generate openapi json file only for test purpose so it was relevant
to create it within test folders but now we want frontend be able to consume
this file so we decide to move it into core root folder.
