Hi, I'm getting some errors when using slack auth.

[efguevara]

Hi, I'm getting some errors when using slack auth.
I can't find the instructions for every provider that you mention @simonprev.

Already have the app installed in slack and my slack user info is registered as shared with the app.
I don't know if I need to add any special scope permissions and if yes which ones?
(https://api.slack.com/scopes)

08:54:56.338 request_id=FgxkhkFEDEXkJFUAAAXB [info] GET /
08:54:56.339 request_id=FgxkhkFEDEXkJFUAAAXB [info] Sent 200 in 898µs
08:54:56.911 request_id=FgxkhmNq1UJ-qfYAAAXh [info] POST /graphql
08:54:56.917 request_id=FgxkhmNq1UJ-qfYAAAXh [info] Sent 200 in 6ms
08:55:07.201 request_id=FgxkiMjIFs3HG_kAAAYB [info] GET /auth/slack
08:55:07.202 request_id=FgxkiMjIFs3HG_kAAAYB [info] Sent 302 in 877µs
08:56:13.004 request_id=FgxkmBrkYyGZjikAAASj [info] GET /auth/slack/callback
08:56:13.393 request_id=FgxkmBrkYyGZjikAAASj [info] Sent 500 in 389ms
08:56:13.393 request_id=FgxkmBrkYyGZjikAAASj [info] Converted error :function_clause to 500 response
08:56:13.396 [error] #PID<0.5392.0> running Accent.Endpoint (connection #PID<0.5391.0>, stream id 1) terminated

Server: 192.168.60.38:4000 (http)
Request: GET /auth/slack/callback?code=XXXXXXXXXXXXXXXXXXX.XXXXXXXXX&state=
** (exit) an exception was raised:
    ** (FunctionClauseError) no function clause matching in Access.get/3
        (elixir) lib/access.ex:265: Access.get("{\"ok\":false,\"error\":\"invalid_auth\"}", "ok", nil)
        (ueberauth_slack) lib/ueberauth/strategy/slack.ex:189: Ueberauth.Strategy.Slack.fetch_auth/2
        (ueberauth_slack) lib/ueberauth/strategy/slack.ex:69: Ueberauth.Strategy.Slack.handle_callback!/1
        (ueberauth) lib/ueberauth/strategy.ex:307: Ueberauth.Strategy.run_callback/2
        (accent) lib/web/controllers/auth_controller.ex:1: Accent.AuthController.phoenix_controller_pipeline/2
        (phoenix) lib/phoenix/router.ex:288: Phoenix.Router.__call__/2
        (accent) lib/web/router.ex:1: Accent.Router.call/2
        (accent) lib/accent/endpoint.ex:1: Accent.Endpoint.plug_builder_call/2

Originally posted by @efguevara in #111 (comment)