Authorization Header added for new slack apps

[bomburr]

Instead, apps must send tokens in the Authorization HTTP header or alternatively as a URL-encoded POST body parameter

https://api.slack.com/changelog/2020-11-no-more-tokens-in-querystrings-for-newly-created-apps

[simonfagerholm]

Duplicates #115

[basroovers]

Why isn't this approved?

[nascentt]

This needs to be approved, psslack is non-functional without compatibility with the api