Basic setup: Merge create and sign-in tabs

[mjs]

Having separate tabs in the setup wizard for creating an account vs signing in to an existing account is causing some confusion. To resolve this, these tabs could be merged into one dynamic "Account" tab.

Proposal:

• If no account is signed in, present the current "create account" form. Underneath the "Create Account button" should be a text saying "Already have an account? Sign in!" where "sign in" is a clickable link.
• If "Sign in" is clicked the controls on the page change to the login form.
• If the wizard is used while an account is signed in, the tab should show the username with a "Sign out" button.
• If "Sign out" is tapped then the tab should show the "create account" form (like on first run).

Inspiration: https://pttrns.com/applications/536#6286