Tell an anonymous user they are not signed in

[wbazant]

Inspired by #632. When submitting locations/reviews anonymously, we ask the user to solve a captcha with the goal of discouraging vandalism. However, it's easy to forget to log in, and we don't explain the rule about only anonymous users having to fill out captchas, so there's potential for confusion.

This issue proposes adding text just under submit buttons in the forms (Location / Review / Add Type / Report):
"Submitting anonymously. CAPTCHA will be required".
or:
"You are not signed in. CAPTCHA will be required".

This will be especially nice after also solving #604 since then someone who's logged out but has an account can go back, log in, and come back to the form.

[wbazant]

Actually, mentioning a captcha will be confusing, since the captcha is sometimes invisible. Instead, just say they are not signed in, and advertise a benefit of some sort?

'Submitting anonymously. Sign in to add as location author'