Suggested capabilities and stripe_express callback url

[jongaegler]

Adds two new features, one is to auth to stripe_express.

Another is to add params to suggest capabilities on org signup shown here https://dashboard.stripe.com/connect-update

[isaacsanders]

@todddickerson Do you mind reviewing this PR? I think this would fix your issue.

[thomasgallagher]

Kindful#2

Extended this PR to allow the user to override the global stripe_express OmniAuth param at the point of redirection to allow projects to mix and match between Stripe Connect Standard (free) and Express (paid) accounts. Also updated the docs.

Please consider merging both into the main repo.

NB. I also spotted 2 specs that were failing that I didn't have time to look at:

Failures:

  1) OmniAuth::Strategies::StripeConnect#authorize_params should not include redirect_uri by default
     Failure/Error: expect(instance.authorize_params[:redirect_uri]).to be_nil
     
       expected: nil
            got: "https://foo.com//auth/stripe_connect/callback"
     # ./spec/omniauth/strategies/stripe_connect_spec.rb:39:in `block (3 levels) in <top (required)>'

  2) OmniAuth::Strategies::StripeConnect#token_params should not include redirect_uri by default
     Failure/Error: expect(instance.token_params[:redirect_uri]).to be_nil
     
       expected: nil
            got: "https://foo.com//auth/stripe_connect/callback"
     # ./spec/omniauth/strategies/stripe_connect_spec.rb:71:in `block (3 levels) in <top (required)>'

[isaacsanders]

It isn't clear that, based on the OmniAuth/OAuth2 library docs, that either of these are necessary for the task you wish to do. Changing the client_options/site value upon configuring your application should be sufficient for changing to the desired endpoint. Additionally, the suggested_capabilities should be listed in the configuration for authorize_params.