signup: Fix the validation for login id.

Change pattern for valid login id from /^[a-zA-Z0-9_]*$/ to
/^[a-zA-Z0-9-]+([_.][a-zA-Z0-9-]*)*$/.

This allows these characters:
* hypen
* underscore and dot, but not for the first or the last character.

And this also removed some lines of code using play-jsmessages, the i18n
module for client side.