Review discrepancies between Grafana and Alertmanager contact points

This is part of the process of unification of Alertmanagers (Mimir and Grafana). Grafana notifiers are derived from the legacy alerting system and have different formats of configuration, use different ways of communicating with the APIs, etc. Therefore, to unify (or not) notifiers we need to review the discrepancies, document them, and then make decisions in each case.

Alertmanager notifiers:

| Notifier | Mimir | Grafana | |
|--------------|-----------|------------|-|
| Alertmanager | 🔲 | :white_check_mark: |
| Ding Ding | 🔲 |:white_check_mark: |
| Discord | :white_check_mark: | :white_check_mark: |
| Email | :white_check_mark: | :white_check_mark: | [link](https://github.com/grafana/alerting/issues/20#issuecomment-1360051784) |
| Google Chat | 🔲 | :white_check_mark: |
| Kafka | 🔲 | :white_check_mark: |
| LINE | 🔲 | :white_check_mark: |
| OPS Genie | :white_check_mark: | :white_check_mark: | [link](https://github.com/grafana/alerting/issues/20#issuecomment-1319321942)
| PagerDuty | :white_check_mark: | :white_check_mark: | [link](https://github.com/grafana/alerting/issues/20#issuecomment-1317895945) |
| Pushover | :white_check_mark: | :white_check_mark: |
| Sensugo | 🔲 | :white_check_mark: |
| Slack | :white_check_mark: | :white_check_mark: |
| SNS | :white_check_mark: | 🔲 |
| MS Teams | 🔲 | :white_check_mark: |
| Telegram | :white_check_mark: | :white_check_mark: | [link](https://github.com/grafana/alerting/issues/20#issuecomment-1300451222)|
| Threema | 🔲 | :white_check_mark: |
| VictorOps | :white_check_mark: | :white_check_mark: |
| Webex | :white_check_mark: | :white_check_mark: |
| Webhook | :white_check_mark: | :white_check_mark: |
| WeChat | :white_check_mark: | 🔲 |
| WeCom | 🔲 | :white_check_mark: |

Global difference:
- In Alertmanager there is a flag `send_resolved` whereas in Grafana it is called `disableResolveMessage`
- Grafana notifiers support images.

Start with the notifier comparison table in this issue and review the listed Mimir and Grafana contact-point formats, API communication differences, and global options such as send_resolved and disableResolveMessage. Document each discrepancy and record whether the notifier should be unified; done means the comparison and decisions cover the listed cases.

Written by the indexing model from the issue text.