Message tweaks to new login and create acct forms

* "Keep me logged in" (bug 47694)
* "[Create my account]" submit button (bug 47700)
* "Why you are creating another account" placeholder (bug 31888)

Bug: 47694
Bug: 47700
Bug: 31888

Change-Id: I7cfa4bb36368277a934144c1724ec437c426eacf
Merge "Message tweaks to new login and create acct forms"