Add scopes

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A basic implementation is already started in source code; but we need to define which scopes will be available, and what data exactly they'll provide.

Here's the begin of a todo list:

• [ ] define all scopes and their contents
• [ ] add user explicit consent for all scopes (including existing one)
• [ ] allow field mapping per configuration

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I've started the work on my fork, I've first included all PR that are still open here. See: https://github.com/galette/plugin-oauth2/pull/4

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Done, see https://github.com/galette/plugin-oauth2/commit/622014fbbdece8b973c83ea33e628f798f0acae5