Send Slack notifications for clades without definition

[joverlee521]

Context

As noted in #73, it may be helpful to have internal notifications of clades that lack clade definitions in our configs. This can serve as a reminder to us to add these clade definitions.

Possible solution

We previously had Slack notifications for clade-without-variant that has since been removed from the pipeline.

Our prepare-data script had an --output-clade-without-variant option that would emit a list of clades to a file. Then we had a Snakemake rule that calls on a custom notify script that will only send the Slack notification if the file was not empty.