LINK-1997 | Merchant improvements

[harriris-vincit]

Description

Makes the following changes:

• A new merchant is created in Talpa every time WebStoreMerchant.paytrail_merchant_id value is changed. This is because Talpa needs to create a new merchant secret mapping when the Paytrail merchant ID is changed and it is recommended by the Talpa devs to create a new merchant in that case.
• WebStoreMerchant.url is no longer editable by users and will default to setttings.LINKED_EVENTS_UI_URL. The URL is used when the "Return to service" button is used in the Talpa checkout UI and therefore it should always be the service's URL.
• RegistrationWebStoreProductMapping references merchant via external_merchant_id (= Talpa's merchant ID) to allow updating it when a merchant has been re-created in Talpa.
• RegistrationWebStoreProductMapping is updated or created when necessary when a payment is created for a signup or signup group. This ensures that the product mapping is up to date in case the merchant has been re-created in Talpa.

Closes

LINK-1997