fix(slack): Left-over fixes from interactive notifications

[luispollo]

Fixes the following:

1. Fix and enable signature verification for Slack callbacks: turns out Spring was messing with the URL-encoded payload for some reason (or so it seems -- at least I couldn't find any other culprits on the path of the request processing). I was unable to find a matching issue against Spring although there were other problems reported with inconsistent URL encoding behavior.
2. Left-over refactored method call that I missed to update before.

[spinnakerbot]

The following commits need their title changed:

• c54a545: Remove token verification in favor of signature verification

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