Allow external validation for season ticket holders

Ex

The validation process for season tickets is the same as the single entries one :

• Special tariffs are created and linked to the external validation through the 'Tariff eligibility verification' interface.
• These tariffs are set with prices on different types of season tickets (Calculated price, Fixed price, Championship).
• In order to buy a season ticket at such tariffs, the internet visitor has to enter a valid username/password to continue purchasing.