Send a Slack DM when user is not in channel (#1144)

# What this PR does

Currently, when a user gets mentioned in an alert group thread and the
user is not in the Slack channel, the Slack bot sends the following to
the channel:

> ⚠️ Tried to ask USER to look at incident. Unfortunately USER is
not in this channel. Please, invite.

This PR changes this behaviour to instead send a direct message to the
user. The message contains a link to the main alert group message in
Slack.

<img width="806" alt="Screenshot 2023-01-17 at 19 25 36"
src="https://user-images.githubusercontent.com/20116910/212996457-02db183f-2041-4998-b743-bd5b6c84b7b5.png">


## Checklist

- [ ] Tests updated (N/A)
- [ ] Documentation added (N/A)
- [x] `CHANGELOG.md` updated

CHANGELOG.md

@@ -11,6 +11,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Allow messaging backends to be enabled/disabled per organization ([#1151](https://github.com/grafana/oncall/pull/1151))
 
+### Changed
+
+- Send a Slack DM when user is not in channel ([#1144](https://github.com/grafana/oncall/pull/1144))
+
 ## v1.1.17 (2023-01-18)
 
 ### Changed

engine/apps/slack/models/slack_message.py

@@ -212,29 +212,7 @@ class SlackMessage(models.Model):
 
                 if slack_user_identity.slack_id not in channel_members:
                     time.sleep(5)  # 2 messages in the same moment are ratelimited by Slack. Dirty hack.
-                    result = sc.api_call(
-                        "chat.postMessage",
-                        channel=channel_id,
-                        text=f":warning: Tried to ask {user_verbal} to look at incident. "
-                        f"Unfortunately {user_verbal} is not in this channel. Please, invite.",
-                    )
-                    SlackMessage(
-                        slack_id=result["ts"],
-                        organization=self.organization,
-                        _slack_team_identity=self.slack_team_identity,
-                        channel_id=channel_id,
-                        alert_group=alert_group,
-                    ).save()
-                    UserNotificationPolicyLogRecord(
-                        author=user,
-                        type=UserNotificationPolicyLogRecord.TYPE_PERSONAL_NOTIFICATION_FAILED,
-                        notification_policy=notification_policy,
-                        alert_group=alert_group,
-                        reason="User is not in Slack channel",
-                        notification_step=notification_policy.step,
-                        notification_channel=notification_policy.notify_by,
-                        notification_error_code=UserNotificationPolicyLogRecord.ERROR_NOTIFICATION_IN_SLACK_USER_NOT_IN_CHANNEL,
-                    ).save()
+                    slack_user_identity.send_link_to_slack_message(slack_message)
         except SlackAPITokenException as e:
             print(e)
         except SlackAPIException as e:

engine/apps/slack/models/slack_user_identity.py

@@ -92,6 +92,49 @@ class SlackUserIdentity(models.Model):
     def __str__(self):
         return self.slack_login
 
+    def send_link_to_slack_message(self, slack_message):
+        blocks = [
+            {
+                "type": "section",
+                "text": {
+                    "type": "plain_text",
+                    "text": "You are invited to look at an alert group!",
+                    "emoji": True,
+                },
+            },
+            {
+                "type": "actions",
+                "elements": [
+                    {
+                        "type": "button",
+                        "text": {"type": "plain_text", "text": "➡️ Go to the alert group"},
+                        "url": slack_message.permalink,
+                        "style": "primary",
+                    }
+                ],
+            },
+            {
+                "type": "context",
+                "elements": [
+                    {
+                        "type": "mrkdwn",
+                        "text": (
+                            f"You received this message because you're not a member of <#{slack_message.channel_id}>.\n"
+                            "Please join the channel to get notified right in the alert group thread."
+                        ),
+                    }
+                ],
+            },
+        ]
+
+        sc = SlackClientWithErrorHandling(self.slack_team_identity.bot_access_token)
+        return sc.api_call(
+            "chat.postMessage",
+            channel=self.im_channel_id,
+            text="You are invited to look at an alert group!",
+            blocks=blocks,
+        )
+
     @property
     def slack_verbal(self):
         return (