Support edits to line item amounts for upcoming recurring contributions

Implement an interface similar to cancel recurring contribution to allow payment processor plugins/extensions to indicate their ability to handle edits to upcoming recurring contributions.

When a contribution_recur is created, currently line items are created for the first contribution. Instead, create them to point to the contribution_recur record itself. When a payment processor IPN callback occurs, change from creating the new contribution by copying the most recent one and its line items to copying the line items and total amount from the contribution_recur record, and as much other info as possible as well (if there are fields that are only present in contribution and not contribution_recur then use them). Ideally, reuse the code for this callback to create the first contribution as well.

Implement a UI to be determined in concert with upcoming changes to Event line item editing (issue to be created) to allow the line items of the upcoming recurring contribution to be edited.

Call a new payment processor method to modify an existing recurring donation. Ensure that this method is implemented in the current version of the Moneris payment processor, and also for the RBC direct debit system implemented for United Church. Change those payment processors so they advertise that they are able to edit line items.

Upgrade:

For each existing contribution_recur record which has upcoming recurring contributions (ie end_date>now() and next_sched_contribution_date>now()), create line items that point to the contribution_recur record by copying the line items pointing to the most recent contribution record in the series.