16874: Content of account_is_setup.text.erb uses Users.UserSetupMailText

The most straightforward solution seems to be for UserSetupMailText to
still be an erb template and is evaluated in the same environment as
as before, but it gets the code from the config file.

Arvados-DCO-1.1-Signed-off-by: Peter Amstutz <peter.amstutz@curii.com>