Migrate from Slacker to official Slack SDK

[tobias-urdin]

There is two issues here:

• The slacker python module is archived and not supported anymore
• The rpm.start web API endpoint is deprecated and behaviour will change later this year

This migrates to the official Slack SDK and starts using the rtm.connect endpoint.

[tobias-urdin]

Closing in favor of https://github.com/scrapinghub/slackbot/pull/229