Signup: unify instant_confirmation and request_confirm

[felixrindt]

In an effort to write smaller PRs, here's the refactored simple signup method as discussed in the design blog.

[coveralls]

coverage: 88.035% (-0.2%) from 88.211% when pulling 38be5d0be5fc291adce95836ab9ff8896060e191 on unify-simple-signup into a985d49478d61c68d3d78650a85089fe055f7a38 on main.

[jeriox]

replaced by new flow/structure concept in #1202