feat(slack)!: add socket mode support

[wass3r]

❤ THANKS FOR HELPING OUT :D

Proposed change

The main change is the support for Slack's Socket Mode which replaces the legacy RTM implementation. Enhancements and upgrades enumerated below:

1. Add Socket Mode support
   • requires slack_app_token in addition to slack_token
2. Remove slack_workspace_token (instead, make sure you have the usergroups:read scope)
3. Update method to get channel info (uses conversation api instead of deprecated method)
4. Changes to make things more consistent

BREAKING CHANGE: slack_verification_token is deprecated by Slack and removed with this PR. It is replaced by slack_signing_secret.

closes #130

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 1042741970

• 60 of 218 (27.52%) changed or added relevant lines in 5 files are covered.
• 7 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.04%) to 35.055%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
core/outputs.go	0	1	0.0%
core/remotes.go	0	1	0.0%
core/configure.go	60	64	93.75%
remote/slack/remote.go	0	20	0.0%
remote/slack/helper.go	0	132	0.0%

Files with Coverage Reduction	New Missed Lines	%
remote/slack/remote.go	1	0%
core/configure.go	2	94.9%
remote/slack/helper.go	4	0%

Totals
Change from base Build 1008478055:	0.04%
Covered Lines:	835
Relevant Lines:	2382

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💛 - Coveralls

[wass3r]

just working on some documentation changes for this .. hang tight :D

[wass3r]

docs are added/modified: target/flottbot-docs#49