feat: use email as username

[romulorosa]

Hi everyone,

I would like to propose a feature which in my oppinion improves the overall UX. The idea is to basically use the email as username so that would allow us to remove one field from the signup page. The idea came from a real use case for a project that I am currently working on at Anaconda.

I would love to hear oppinions and feedbacks about this approach and also ideas on how to improve the implementation, that is the very first time that I am contributing to this project (which by the way is awesome, thanks!)

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[moraisaugusto]

It would be nice to have this feature.