Clarify user signup
Clarify user signup

Why:

Some of the user testing discovered a couple confusing spots in the user
signup process.

This change addresses the need by:

* Adding message about using a previous account.

* Removing the password placeholder.

* Adding mention of the password length requirement.

Trello: https://trello.com/c/pLVIcIWn/135-signup-page

MAPC/developmentdatabase,MAPC/developmentdatabase,MAPC/developmentdatabase,MAPC/developmentdatabase