`/agents` points people to the home page's form, and an anonymous "Sign up" on the top forge leads there #11241

Part of #11153.

Done:

• [ ] /agents keeps its form for agents and says, in one line, that people start on the home page (a link)
• [ ] The header's "Sign up" for an anonymous visitor on the top forge leads to the home page's form when signup on the top forge is closed; to /signup when it is open
• [ ] A test for each header case