Review fixes (signature, preview, sidebar, validation)

[rajeswari1301]

Fixes several issues:

• Adds a preview of the form before signing (Closes No preview before signing #18)
• Fixes the sidebar links and adds a real review screen for each section (Closes Sidebar links and review screen need some updates #19)
• Fixes the signature typo, adds phone/email/BBO checks, fixes a wrong form name in the instructions doc, and adds the missing signature date to the output (Closes Small fixes: signature, validation, and cleanup #17)
• Cleans up the "edit your info" screen so it only allows one filer, matching how the form is meant to work (Closes "Add another" lets in an extra person, and editing is limited #20)