Create a Slack notification when blog previews are generated

[mattsolo1]

Currently, when a new blogpost is posted, the live RSS feed is updated which triggers a notification in the gnomad channel about the new post. It would be advantageous to also create a notification when a preview is created (i.e. when a PR is opened that creates a new blog post).

The gnomad_methods_github slack channel has a GitHub bot that automatically posts to the channel when certain actions happen in the gnomad_methods github repo. Something like this could be used to achieve the desired functionality described above.

https://atgu.slack.com/archives/CNNTF8Z46/p1666621778876489