enhance(slack)!: fix workspace token; switch to signing secret

[wass3r]

❤ THANKS FOR HELPING OUT :D

Proposed change

Previously we used a separate token (OAuth Acccess Token via slack_workspace_token) because it gave access to the usergroups in your workspace. With the permissions changes that Slack made not so long ago, you can now request a usergroup:read permission for your regular token. This PR drops the slack_workspace_token requirement if you want to leverage the usergroup functionality.

BREAKING CHANGE:
This PR will change the slack_verification_token and replace it with the recommended Signing Secret via the new slack_signing_secret property. If you have the Events API version of the bot deployed you will need to make this change if you are using a version of the bot with this change implemented.

Types of changes

What types of changes is this pull request introducing to flottbot? Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

You can fill this out after creating your PR. Put an x in the boxes that apply

• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

[coveralls]

Pull Request Test Coverage Report for Build 715211973

• 6 of 90 (6.67%) changed or added relevant lines in 6 files are covered.
• 8 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.7%) to 35.327%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
core/remotes.go	0	2	0.0%
core/outputs.go	0	3	0.0%
remote/slack/remote.go	0	8	0.0%
utils/access_check.go	0	15	0.0%
remote/slack/helper.go	0	56	0.0%

Files with Coverage Reduction	New Missed Lines	%
remote/slack/helper.go	3	0%
utils/access_check.go	5	64.58%

Totals
Change from base Build 704086029:	-0.7%
Covered Lines:	821
Relevant Lines:	2324

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💛 - Coveralls