See https://meta.discourse.org/t/discourse-email-messages-are-incorrectly-threaded/233499
for thorough reasoning.
This commit changes how we generate Message-IDs and do email
threading for emails sent from Discourse. The main changes are
as follows:
* Introduce an outbound_message_id column on Post that
is either a) filled with a Discourse-generated Message-ID
the first time that post is used for an outbound email
or b) filled with an original Message-ID from an external
mail client or service if the post was created from an
incoming email.
* Change Discourse-generated Message-IDs to be more consistent
and static, in the format `discourse/post/:post_id@:host`
* Do not send References or In-Reply-To headers for emails sent
for the OP of topics.
* Make sure that In-Reply-To is filled with either a) the OP's
Message-ID if the post is not a direct reply or b) the parent
post's Message-ID
* Make sure that In-Reply-To has all referenced post's Message-IDs
* Make sure that References is filled with a chain of Message-IDs
from the OP down to the parent post of the new post.
We also are keeping X-Discourse-Post-Id and X-Discourse-Topic-Id,
headers that we previously removed, for easier visual debugging
of outbound emails.
Finally, we backfill the `outbound_message_id` for posts that have
a linked `IncomingEmail` record, using the `message_id` of that record.
We do not need to do that for posts that don't have an incoming email
since they are backfilled at runtime if `outbound_message_id` is missing.
