Unisender GO raises AnymailWebhookValidationFailure on some webhook events

[MikeVL]

Reporting an error? It's helpful to know:

• Anymail version v11.1
• Unisender GO
• Versions of Django 4.1, requests, python 3.12
• EventType SENT, DELIVERED fine, but OPENED, CLICKED raises exception
• Fix AnymailWebhookValidationFailure on some events #397 (tested in prod)

[medmunds]

I've managed to track down the underlying problem. It affects any Unisender Go webhook payload that includes a / in the JSON:

• All "clicked" events (the clicked "url" has slashes in "https://...")
• All "opened" events if you have enabled "Additional information about delivery" (the "user_agent" typically includes slashes)
• All events—of any type—if the message was sent with metadata values that include a slash (e.g., msg.metadata = {"date_joined": "2022/01/22"})

The problem is that Unisender Go's JSON serialization (unnecessarily) backslash-escapes forward slashes:

{"url":"https:\/\/example.com\/"}

And Python's does not, meaning the md5 hashes don't match.

The proposed solution in your PR does solve the problem. While updating the tests, I uncovered several other issues in Anymail's current Unisender Go webhook implementation (and several missing test cases). So I'm going to open a new PR that pretty much rewrites that webhook and its tests.