Add configuration options for when the Slack channels/conversations are created

[stuartcampbell]

Add configuration option(s) for start-experiment to kick off creation of Slack channels.
This could be a flag we store in the Beamline database collection

Also add a configuration option to indicate if the channels (conversations) should be auto created.

These changes might not be actually in this codebase but I created the issue here so not to forget.