This commit adds 4 new payload fixtures to the Slack
integration for messages we don't process properly yet:
- Messages with code blocks.
- Messages with overlapping text formatting (e.g, bold
and italic).
- Messages with formatted mentions.
- Messages with quote blocks.
The follow-up plan to add support for these kind of
Slack messages is tracked in #30827.
Part of #30465.
This updates the Slack webhook integration to handle the Slack Events
API[1], while maintaining backwards compatibility with Slack's legacy
Outgoing Webhook service.
The Events API introduces the "challenge" handshake[2] to verify and
add a new webhook URL for them to call. This commit adds a handler for
the challenge handshake.
Additionally, this commit reformats incoming payloads using the Slack
text reformatter from `slack_message_conversion.py`. There is some
duplicative code here because of the difference in Slack export data
and Slack's webhook payload.
Part of #30465
[1]: https://api.slack.com/apis/events-api#using-events-api
[2]: https://api.slack.com/apis/events-api#handshake
All webhook fixtures in zerver/fixtures/<webhook_name> have now
been moved to dedicated webhook-specific directories under
zerver/webhooks/<webhook_name>/fixtures, where <webhook_name> is
the name of the webhook.
