Use official slack/slack-go library

[nitrocode]

what

• Use official slack/slack-go library
• Removed a couple tests

why

• Deprecate previous nlopes/slack
• Good security practice to use a well-maintained and official dependency
• Couple mock tests were removed because of the inability to test them with variadic params (see issue)

references

• Closes Use official slack/slack-go client #2667
• official https://github.com/slack-go/slack
• old fork https://github.com/nlopes/slack
• mocking lib https://github.com/petergtz/pegomock
• lyft/atlantis is also affected
• Variadic Issue? not really sure. petergtz/pegomock#112

notes

• Needed to regenerate the mocks using pegomock which was difficult. It would be good to replace pegomock with something more up to date. It's also questionable how much value is generated from a mocked test for a function that wraps an upstream library.

[nitrocode]

@nishkrishnan if you have time, I'd love a second look on these tests

[jamengual]

lgtm