Expiration date

Problem/Motivation

Purchasable gift cards usually have expiration date.

Proposed resolution

1. Add expiration_date to gift card. Optional field.
2. Gift card type should have an option to set number of days (months/years) for how long gift cards are valid. Optional field.
3. On purchase (on paid) expiration_date should be calculated: now() + number of configured days (months/years) for this type. This calculation should properly interpret months (which are not 30 days in every case) and years (which are not 365 days in every case).
4. When gift card is expired - display appropriate error for end user if tries to use it.
5. Allow a general expiration duration, for example due to legal regulations (e.g. 3 years) as default