For "sign in" and "sign up", provide hints about existing accounts on other sites

[cellio]

If you click "sign in" it asks you for an email address and password (quite reasonably), and if you click "sign up" it asks for info and offers to create an account for you. But if you already have an account on a different Codidact site, things are a little confusing: "sign up" won't work because that account already exists, and "sign in" does work but since you've never been here before you might not think to try it.

Could we add the following notices to these pages, above the forms?

Sign up: "If you have an account on another Codidact site, do not create a new account here. Instead, [sign in](link to sign-in page) with that account."

Sign in: "Your username and password are the same on all Codidact sites."