Move slack advanced exporter into the mmetl tool

[coltoneshaw]

Look at moving this tool within mmetl to prevent switching between various tools. Also, allows us to improve the flow

https://github.com/grundleborg/slack-advanced-exporter

[wetneb]

This tool doesn't seem actively maintained anymore, so it's blocking for things such as #57.

It would also make for a more streamlined UX if we could have one less tool in the migration pipeline described here:
https://docs.mattermost.com/onboard/migrate-from-slack.html