Test error messages given with blank signup
Test error messages given with invalid email signup
Test error messages given with mismatched password/confirmation email signup
Change blank "password digest" error to say "password"
Remove obsolete error message from test
Test for correct title and welcome text after saving user
Make User#create tests fail

FOR GREAT PEDAGOGY!
Revert "Make User#create tests fail"

Make tests pass again.
This reverts commit 0e38e5b2806bd04232a7190df28ec752f6cefb20.