Use cancel_reason for RCUR mandates

[bjendres]

Historically, CiviSEPA used a note entity to store the cancel reason that was entered by the user upon cancellation. The reason is, that, the civicrm_contribution_recur table didn't have a cancel_reason column, like the civicrm_contribution table. See also #409.

Recently, however, this column has been added, so we should start using it.

Implementing would include:

• using the new column
• migrating the old data into that column
• expose this in the reports
• keep backwards compatibility?

[lbl-knts]

Record the cancel reason in the (core) contribution field seems then to be searchable for fundraisers, who want to know why the customers cancel their recurring contributions. Would be valuable feature!