[!!!][FEATURE] Extend price options functionality

[derhansen]

The current price option feature is not flexible enough to allow mutliple prices for an event. It is not possible to name the price options and it is currently always required to have a validity for the price option. The getters getActivePriceOptions() and getCurrentPrice() are specially crafted to return price options and the price based on the validity of a price option.

The price option functionality will therefore be extended as following

• Each price option has a "Title", a "Description" and a "Price"
• Support for fe_group, start- and stop-time
• Allow sorting of price options in event record

The event domain model will be extended/changed as following:

• Getter and setter for price options. Getter returns price options sorted by sort order in backend
• getActivePriceOptions() will be removed
• Getter getCurrentPrice() which either returns the price field (if no price option is defined) or the price of the lowest price option.

The registration model will be extended as following:

• New field price where the final price is saved (Tab: Payment)
• New field price_option, where the selected price option is saved (Tab: Payment)

The saveRegistration() will be extended as following:

• Determine and save the price for the event. Either price field if no price option is given or the price of price_option if given
• Save the selected price_option
• Ensure only available/valid price options can be saved
• Add PSR-14 event to modify price before persistence