Update the slack-client dependency

[ryanguill]

We currently use https://www.npmjs.com/package/slack-client but it has been deprecated and now we should be using https://www.npmjs.com/package/@slack/client

https://slackapi.github.io/node-slack-sdk/changelog

I have no idea how much this is going to break but we're pretty far behind at this point.

[ryanguill]

OT but it really grinds my gears that they pointed the old package to the new github repo.

[atuttle]

[yamishi13]

I can take a look at this if it's ok

[ryanguill]

sure thing @yamishi13! I am hopeful that everything that would need to be changed would be confined to main.js and plugins/core.js, but I don't know that for sure. Feel free to ping me if you have any questions or need any help. Thanks!