Update Slack webhook integration to use the new Slack app instead of legacy custom integration

[PieterCK]

📄 Overview

Currently, our Slack integration uses the outdated 'Outgoing WebHooks' custom integration feature to configure the webhook integration.

from Slacks' Outgoing Webhooks page:

Please note, that this is a legacy custom integration - an outdated way for teams to integrate with Slack. These integrations lack newer features and they will be deprecated and possibly removed in the future. We do not recommend their use. Instead, we suggest that you check out their replacement: Slack apps.

The recommended way to set up a webhook integration with Slack is to use Slack Apps Event API.

This PR aims to report which files and features would be impacted if we decided to migrate to the Slack Apps for setting up webhook integrations.

Relevant links:
Migrating from legacy integrations

CZO thread: here

Documentation

Setting up webhook integration through the Events API would require guiding the user through creating the new Slack App and setting up Events API.

current doc:

Test Fixtures

Using Events API, we would subscribe to the message.channels event to replicate the current behavior of the legacy Outgoing Webhook. The new payload provides similar data but is in a different format from the current test fixtures.

While the legacy outgoing webhook only worked for public channels, the Events API supports extensive options such as sending messages from DMs, private channels, and also payload from dozens of other event types. Adding support for the newer features from Events API would require adding more test fixtures

current test fixture for a message:

token=aotJImaVOVEVWssawxmegDWt&team_id=T3W8CTX0F&team_domain=gsoc2017&service_id=133126759712&channel_id=C3W8CTZFZ&timestamp=1485947263.000010&user_id=U3W8CTX2T&user_name=slack_user&text=test

the same payload from Events API:

{
    "token": "one-long-verification-token",
    "team_id": "T123ABC456",
    "api_app_id": "A123ABC456",
    "event": {
        "type": "message",
        "channel": "C123ABC456",
        "user": "U123ABC456",
        "text": "Live long and prospect.",
        "ts": "1355517523.000005",
        "event_ts": "1355517523.000005",
        "channel_type": "channel"
    },
    "type": "event_callback",
    "authed_teams": [
        "T123ABC456"
    ],
    "event_id": "Ev123ABC456",
    "event_time": 1355517523
}

[zulipbot]

Hello @zulip/server-integrations members, this issue was labeled with the "area: integrations" label, so you may want to check it out!

[alya]

Thanks, @PieterCK ! You can use Zulipbot to add labels to issues you file.