Fix rank validation in admin account creation
Move messages to their own class

- Use {0} and {1} in translations for replacements instead of # and $
- Add support for arbitrarily number of replacements
- Add a simple message system. Messages are not displayed yet.
Use login and logout instead of log_in and log_out

The class names are also LoginPage and LogoutPage, not LogInPage and
LogOutPage. This fixes an issue on Linux and Mac OSX were the page was
not found, which was hidden on Windows because of the case insensitive
file system and the case insensitive class names.