Alerting: Fix notification channel migration

[papagian]

What this PR does / why we need it:
Starting from grafana 7.2 notification channel secure settings are stored encrypted in the database. But for the existing alert notification channels, there was no automatic migration of storing sensitive settings encrypted. Therefore there can still exist such unencrypted secure settings (if the channel is not saved since upgrading to >= 7.2)
The dashboard alert migration when enabling the new alerting system takes cares of it and encrypts those settings so that all the secure settings are stored encrypted in the Alertmanager configuration stored in the database.
However, there was a bug in case such a migrated receiver was used in multiple contact points because we used to delete this setting. As a result, the secure setting was stored only the 1st time and was missing from all the rest.

Example of such problematic configuration

The slack receiver in autogen-contact-point-default has url property while not in autogen-contact-point-1

{
  "template_files": {},
  "alertmanager_config": {
    "route": {
      "receiver": "autogen-contact-point-default",
      "routes": [
        {
          "receiver": "autogen-contact-point-1",
          "matchers": [
            "rule_uid=\"-lmj0SI7zm\""
          ]
        }
      ]
    },
    "templates": null,
    "receivers": [
      {
        "name": "autogen-contact-point-default",
        "grafana_managed_receiver_configs": [
          {
            "uid": "m_mC0IInkz",
            "name": "slack receiver",
            "type": "slack",
            "disableResolveMessage": false,
            "settings": {
              "autoResolve": true,
              "httpMethod": "POST",
              "iconEmoji": "",
              "iconUrl": "",
              "mentionGroups": "",
              "mentionUsers": "",
              "recipient": "#test-alerts",
              "severity": "critical",
              "token": "",
              "uploadImage": true,
              "username": ""
            },
            "secureFields": {
              "url": true
            }
          }
        ]
      },
      {
        "name": "autogen-contact-point-1",
        "grafana_managed_receiver_configs": [
          {
            "uid": "3lmC0SSnz",
            "name": "webhook",
            "type": "webhook",
            "disableResolveMessage": false,
            "settings": {
              "autoResolve": true,
              "httpMethod": "POST",
              "password": "",
              "severity": "critical",
              "uploadImage": true,
              "url": "http://localhost:3010",
              "username": ""
            },
            "secureFields": {}
          },
          {
            "uid": "q_iCAII7zz",
            "name": "slack receiver",
            "type": "slack",
            "disableResolveMessage": false,
            "settings": {
              "autoResolve": true,
              "httpMethod": "POST",
              "iconEmoji": "",
              "iconUrl": "",
              "mentionGroups": "",
              "mentionUsers": "",
              "recipient": "#test-alerts",
              "severity": "critical",
              "token": "",
              "uploadImage": true,
              "username": ""
            },
            "secureFields": {}
          }
        ]
      }
    ]
  }
}

Special notes for your reviewer:
For reproducing the error one has to create an entry in the alert_notification table containing such an un-encrypted property (eg url property under the settings column for a slack notifier), use this channel in more than one alerts and enabling ngalert.

[grafanabot]

The backport to v8.1.x failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new branch
git switch --create backport-38983-to-v8.1.x origin/v8.1.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x b56bf83c19fc8f2dafdc745f02283f7ff27a387e
# Push it to GitHub
git push --set-upstream origin backport-38983-to-v8.1.x
git switch main
# Remove the local backport branch
git branch -D backport-38983-to-v8.1.x

Then, create a pull request where the base branch is v8.1.x and the compare/head branch is backport-38983-to-v8.1.x.