Add progress indicator to sign-up flow (#24545)

app/controllers/auth/setup_controller.rb

app/helpers/application_helper.rb

app/javascript/styles/mastodon/forms.scss

app/views/auth/registrations/new.html.haml

app/views/auth/registrations/rules.html.haml

app/views/auth/setup/show.html.haml

app/views/auth/shared/_links.html.haml

app/views/auth/shared/_progress.html.haml

config/locales/en.yml

config/locales/simple_form.an.yml

config/locales/simple_form.ar.yml

config/locales/simple_form.ast.yml

config/locales/simple_form.be.yml

config/locales/simple_form.bg.yml

config/locales/simple_form.ca.yml

config/locales/simple_form.ckb.yml

config/locales/simple_form.co.yml

config/locales/simple_form.cs.yml

config/locales/simple_form.cy.yml

config/locales/simple_form.da.yml