Update Slack provider to use v2 API and allow Bot tokens

[jbrooksuk]

This PR changes a few things with Slack:

1. Switches to the v2 API.
2. Adds a asBotUser() method that switches the type of scopes requested. Slack has two kinds of tokens; xoxb (bot) and xoxp (user). When using asBotUser() the user() method will return an empty user, but a bot token will still be available.

[ataldev]

We should be able to use user_scopes and bot scopes in the same redirect.
For example user_scopes=identity.email&scopes=commands
Currently, to achieve this, the asBotUser can not be utilized. And we should use something like:

return Socialite::driver('slack')->scopes(['identity.email'])->with(['scopes' => 'commands'])->redirect()

Which is not ideal.