[4.x] Allow customisation of authentication logic

[joelbutcher]

This PR

• moves the logic used for handling request errors on return back from an OAuth flow to an action
• moves the authentication logic used for creating and authenticating users returning from an OAuth flow into an action
• Allows developers to override both of these via overriding the authenticatesOauthCallbackUsing and handlesOAuthCallbackErrorsUsing bindings:

Socialstream::authenticatesOauthCallbackUsing(AuthenticateOauthCallback::class);
Socialstream::handlesOAuthCallbackErrorsUsing(HandleOauthCallbackErrors::class);