Explore integration with slack

[mrchrisadams]

Almost every instance of constellate is linked to an existing slack workspace.

As such it would make sense to allow users to sign in via slack, an to have something like a 'blessed' channel for members who want to be opt-in to being in a constellate directory.

This would allow an explicit way for a user of slack who is in the channel, to also exist inside the directory, so they are discoverable, and can share richer, more easilty searchable profiles than slack's own directory features support.

The rough plan

• figure out how to sign in to a workspace from python via slack
• add an importer that runs through a membership of a given channel, and imports all the current users are have not already been imported
• figure out a clear way for folks to make sure they no longer appear in the directory, and delete any trace

This would be sufficient for a first pass.