Unify registration user experience across mix and rebar3_hex

[starbelly]

Background

In has been agreed by this working group we should provide a unified registration experience across mix and rebar3_hex. See #5 for more details.

Tasks

The following tasks must be completed to resolve this issue.

• Update hex to follow the flow proposed in RFC : UX for user registration in hex and rebar3_hex #5
• Update rebar3_hex to follow the flow proposed in RFC : UX for user registration in hex and rebar3_hex #5

Related agenda items, issues, and pull requests.

• RFC : UX for user registration in hex and rebar3_hex
• Minutes for 2020-11-06

[starbelly]

It's been sometime, is mix hex still interested in this? I know we want to make web auth the default, but the current auth method would not go away any time soon, if ever.