Unisender Go: Fix status tracking webhook and tests.

- Fix signature checking to avoid false validation errors
  on webhook payloads including `/` (including all "clicked"
  and most "opened" events). And in general, avoid depending
  on specific details of Unisender Go's JSON serialization.
  (Fixes #398.)
- Handle "use single event" webhook option (which has a different
  payload format).
- Verify basic auth when Anymail's WEBHOOK_SECRET is used.
  (This is optional for Unisender Go, since payloads are signed,
  but it needs to be checked when enabled.)
- Treat "soft_bounced" events as "deferred" rather than "bounced",
  since they will be retried later.
- Update validation error to reference Project ID if the webhook
  is configured for a specific project.
- Expose Unisender Go's delivery_status code and unsubscribe form
  comment as Anymail's normalized event.description.
- Update webhook tests based on actual payloads and add several
  missing tests.
- Update docs to clarify webhook use with Unisender Go projects.

tests/test_files/unisender-go-tracking-test-payload.json.raw

@@ -0,0 +1 @@
+{"auth":"b3cb4d6aef9d07095805c39e792e0542","events_by_user":[{"user_id":5960727,"project_name":"Testing","project_id":"6862471","events":[{"event_name":"transactional_email_status","event_data":{"job_id":"1sn15Z-0007Le-GtVN","email":"unisendergo@anymail.dev","status":"clicked","event_time":"2024-09-08 19:56:41","url":"https:\/\/example.com","delivery_info":{"user_agent":"Mozilla\/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit\/537.36 (KHTML, like Gecko) Chrome\/128.0.0.0 Safari\/537.36","ip":"66.179.153.169"}}}]}]}