Not able to receive Slack notifications for oncall reminders

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> @minhaz1 could you help here as I believe from https://github.com/linkedin/oncall/issues/394 post that you can provide some guidance here too! Thank you.

Hi, I am able to setup the oncall roster and schedule for my team (screenshot attached).

**I have also made following changes to ./configs/config.yaml**
```
notifier:
skipsend: False
notifications:
default_modes:
- "slack"
messengers:
- type: slack
webhook: "https://hooks.slack.com/triggers/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"
```

**However, I am still NOT able to receive notifications.**
My Database enteries looks like this -

```
mysql> select * from notification_setting;
+----+---------+---------+---------+---------+-------------+------------------+
| id | user_id | team_id | mode_id | type_id | time_before | only_if_involved |
+----+---------+---------+---------+---------+-------------+------------------+
| 1 | 1 | 1 | 4 | 1 | 86400 | 1 |
| 2 | 1 | 1 | 4 | 1 | 604800 | 1 |
| 3 | 2 | 1 | 4 | 1 | 86400 | 1 |
| 4 | 2 | 1 | 4 | 1 | 604800 | 1 |
| 7 | 4 | 1 | 4 | 1 | 86400 | 1 |
| 8 | 4 | 1 | 4 | 1 | 604800 | 1 |
| 9 | 5 | 1 | 4 | 1 | 86400 | 1 |
| 10 | 5 | 1 | 4 | 1 | 604800 | 1 |
| 11 | 6 | 1 | 4 | 1 | 86400 | 1 |
| 12 | 6 | 1 | 4 | 1 | 604800 | 1 |
| 13 | 7 | 1 | 4 | 1 | 86400 | 1 |
| 14 | 7 | 1 | 4 | 1 | 604800 | 1 |
| 15 | 8 | 1 | 4 | 1 | 86400 | 1 |
| 16 | 8 | 1 | 4 | 1 | 604800 | 1 |
| 17 | 9 | 1 | 4 | 1 | 86400 | 1 |
| 18 | 9 | 1 | 4 | 1 | 604800 | 1 |
+----+---------+---------+---------+---------+-------------+------------------+
```

```
mysql> select * from event;
+----+---------+---------+-------------+---------+---------+------------+------------+------+
| id | team_id | role_id | schedule_id | link_id | user_id | start | end | note |
+----+---------+---------+-------------+---------+---------+------------+------------+------+
| 1 | 1 | 1 | 1 | NULL | 7 | 1693794600 | 1694795400 | |
| 2 | 1 | 2 | 1 | NULL | 9 | 1693794600 | 1694795400 | |
| 31 | 1 | 1 | 7 | NULL | 9 | 1695004200 | 1696213800 | NULL |
| 32 | 1 | 1 | 7 | NULL | 6 | 1696213800 | 1697423400 | NULL |
| 33 | 1 | 1 | 7 | NULL | 5 | 1697423400 | 1698633000 | NULL |
| 34 | 1 | 1 | 7 | NULL | 8 | 1698633000 | 1699842600 | NULL |
| 35 | 1 | 1 | 7 | NULL | 3 | 1699842600 | 1701052200 | NULL |
| 36 | 1 | 1 | 7 | NULL | 7 | 1701052200 | 1702261800 | NULL |
| 37 | 1 | 1 | 7 | NULL | 9 | 1702261800 | 1703471400 | NULL |
| 38 | 1 | 2 | 8 | NULL | 6 | 1695004200 | 1696213800 | NULL |
| 39 | 1 | 2 | 8 | NULL | 5 | 1696213800 | 1697423400 | NULL |
| 40 | 1 | 2 | 8 | NULL | 8 | 1697423400 | 1698633000 | NULL |
| 41 | 1 | 2 | 8 | NULL | 3 | 1698633000 | 1699842600 | NULL |
| 42 | 1 | 2 | 8 | NULL | 7 | 1699842600 | 1701052200 | NULL |
| 43 | 1 | 2 | 8 | NULL | 9 | 1701052200 | 1702261800 | NULL |
| 44 | 1 | 2 | 8 | NULL | 6 | 1702261800 | 1703471400 | NULL |
+----+---------+---------+-------------+---------+---------+------------+------------+------+

```

Start with ./configs/config.yaml and compare the notifier, notifications, and messengers settings with the notification_setting and event rows shown. Trace the reminder delivery path for the configured Slack mode and verify that a scheduled reminder is delivered; the issue is done when on-call reminders arrive for the configured users.

Written by the indexing model from the issue text.