Purpose
=======
Be able to send/receive Facebook messages in Discuss.
The code must be generic, to be able to send messages on other medias,
in the future.

Specifications
==============
Features
--------
We want to be able to send messages from Odoo to Facebook and to
receive the Facebook messages in Odoo.

We want to specify operators per page. So, a list of users will receive
the messages of the Facebook page and will be able to respond.

Facebook webhook
----------------
Facebook manage events (as new message) with webhook. You specify an URL
in your application setting, and when you receive a new message on your
page, Facebook will perform a HTTP request on the endpoint you specify
with the message information.

The steps are
1. You create a Facebook application, and you set your callback URL to
   `<web_base_url>/social_facebook/webhook`
2. You connect your Facebook account with Odoo
   (so we have an access token per page)
3. Each page make a HTTP request to register itself to the webhook
   (to receive the new messages)
4. When a new message is received on a page, Facebook perform a HTTP
   request with the page ID.

IAP
---
If the user use IAP, a callback URL will be passed to IAP during the
authentication. We save it in the IAP database.
So, when IAP will receive a new event, we look at the Facebook page ID
(in the data we received from Facebook), and we give the event to the
right callback URL. Note that multiple Odoo database might be linked to
the same Facebook page, that Facebook send notifications in batch(so we
can receive notifications of different page/database in same request).

Signature
---------
In order to prevent malicious users from impersonating Facebook sign
each message with the APP secret. If the events are passed thought IAP,
the signature is checked when IAP receive the message and then IAP sign
each request to each database with the shared secret passed during the
authentication (this key is build with the database secret).

So, each message is signed and can not be impersonate.

Operators
--------------
To not spam all operators with all social messages, we want to choose only one operators to respond to the Facebook contact.


The order of priority to choose an operator is
1. An online operator who is not in an active social channel
2. The online operator who has the lowest number of active social channel
3. An offline operator who is not in an active social channel
4. The offline operator who has the lowest number of active social channel

A social channel is considered as "active" if one message has been sent in the last 30 minutes.

We take offline operators if nobody is available to not loose the conversation and to be able to respond later to the customer.

Task 2124457
See odoo/odoo/pull/45493