Slack Events API Update

[GLOFonseca]

Cleanup

What should be cleaned up or changed:
On February 24, 2021 the Events API from Slack will be updated.
We need to check if anything would impact the Hellper bot.

Provide any links for context:
https://api.slack.com/changelog/2020-09-15-events-api-truncate-authed-users

[tiagotor]

we are using the authed_users into events tests (hellper/internal/handler/events_test.go).

In every callback we expect to get a authed_users:

"authed_users":["UEVHT00G0"]

We need to change it to use the apps.event.authorizations.list method.