Register Contributions

Created the framework to contribute to the register form.
Added doc on how to use it in the readme
refactor the register page to use only "contributions" even for its own aka Account Info and Optionnal Informations

Some bug fix and added fixmes

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I'm just curious, what useful thing can the other modules do with this, *in the frontend*? (I guess I'll get to see shortly with the other PRs)