Before this patch, the module was designed after the [deprecated Mailgun webhooks][3]. However Mailgun had the [events API][2] which was quite different. Modern Mailgun has deprecated those webhooks and instead uses new ones that include the same payload as the events API, so you can reuse code.
However, this was incorrectly reusing the code inversely: trying to process the events API through the same code prepared for the deprecated webhooks.
Besides, both `failed` and `rejected` mailgun events were mapped to `error` state, but that was also wrong because [`mail_tracking` doesn't have an `error` state][1].
So the logic of the whole module is changed, adapting it to process the events API payload, both through controllers (prepared for the new webhooks) and manual updates that directly call the events API.
Also, `rejected` is now translated into `reject`, and `failed` is translated into `hard_bounce` or `soft_bounce` depending on the severity, as specified by [mailgun docs][2]. Also, `bounced` and `dropped` mailgun states are removed because they don't exist, and instead `failed` and `rejected` properly get their metadata.
Of course, to know the severity, now the method to obtain that info must change, it' can't be a simple dict anymore.
Added more parameters because for example modern Mailgun uses different keys for signing payload than for accessing the API. As there are so many parameters, configuration is now possible through `res.config.settings`. Go there to autoregister webhooks too.
Since the new webhooks are completely incompatible with the old supposedly-abstract webhooks controllers (that were never really that abstract), support for old webhooks is removed, and it will be removed in the future from `mail_tracking` directly. There is a migration script that attempts to unregister old webhooks and register new ones automatically.
[1]: f73de421e2/mail_tracking/models/mail_tracking_event.py (L31-L42)
[2]: https://documentation.mailgun.com/en/latest/api-events.html#event-types
[3]: https://documentation.mailgun.com/en/latest/api-webhooks-deprecated.html
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