Signin with slack support

[jwright]

This PR adds Sign in with Slack support via an adapter pattern.

Since Slack responds with a different JSON structure with identity scopes and app scopes, this PR introduces an adapter for each different scope so it has a common API to respond with user_info, raw_info, team_info, and the uid.

For example, an app requesting the identity.basic scope needs to call the /api/users.identity whereas the non-identity scopes needs to call the /api/auth.test method. These two methods return different JSON responses with different structures.

*This PR will not merge with the current master. This is because another PR that added sign in with Slack support eliminated a lot of the information provided by the strategy. Information such as user_id and team_id which I believe will break a lot of current installs. I recommend replacing it with this PR since it does not eliminate the fidelity of the current information that is provided by the strategy. *

[Petercopter]

I'm not sure what happened... I was using the code from @Z-productions, and it was working for me, but now that's its merged, master isn't working for me, it's acting differently than it was.

What can we do to get this moving? I would love to get Slack sign in working...

[jwright]

@Petercopter I am not sure what error(s) you are seeing now but feel free to try this fork. It has been working for me (without any changes to code from this commit) in production for a month now. You may need to adjust the signatures from the oauth response.

[ys]

This fork worked great for me to get the sign in with slack!
I would love to see this hit a gem release:)

[yhirano55]

👍

[julienma]

I also confirm I had to use this branch with #34 (which works fine) instead of #32 (which didn't work).

[gfortaine]

Same here.

[kandadaboggu]

This PR breaks the old uid generation logic. In the original logic, uid was scoped by team id. I have submitted a PR to @jwright to address the issue: jwright#2

[jwright]

@kandadaboggu I noticed you closed the PR without any comment. Is this a resolved issue?

[joshuatobin]

@kmrshntr Any thoughts on this PR and cutting a new gem?