Authentication system overhaul. (part 2)

- Rename TokenConfirmEmail to TokenActivationEmail.
- Refer user account by email instead of ObjectId in activation tokens.
- Use only secret key for account activation, no user id.
- Move 'confirm_email' code into 'register' handler.
- Send first activation letter directly from 'register' handler without
  redirection tricks.
- Add UserGroup.findId method to clear register/activation code.
- Drop unused settings for email confirmation and password reset
  timeouts. We use constants for that.
- Add setting "Validated user group" to use on account activation.
- Add "Project Name" to subject of account activation letter.
- Registration: Send translated error messages instead of i18n paths.
- Registration: Validate ReCAPTCHA only if all other fields are valid.
- Registration: Correctly switch error states and messages on inputs.
- Validate nick format on server:users.auth.check_nick
- Correct translation texts.