Slack: support images in alerts #801

[rozetko]

Slack doesn't support images uploading through webhooks, only with API. This PR adds new slack alert type which can send alerts to Slack with images

Changes:

• new @slack/web-api dep
• new configuration options:
  • HASTIC_SLACK_API_TOKEN: https://slack.com/intl/en-ru/help/articles/115005265703-Create-a-bot-for-your-workspace
  • HASTIC_SLACK_NOTIFICATION_CHANNEL: name of channel to send alerts to
• new slack alert type
• return Buffer instead of base64-string from loadImage so that we can use it with files.upload Slack-API method
• new related wiki page: https://github.com/hastic/hastic-server/wiki/Slack

Other changes:

• rename makeNotification method to generateNotification