Add comment/note for scheduled payment to pot

This idea was referenced in another thread (Direct Debit Pot / Reference for scheduled payments) but was skimmed over by the responses.

The idea is simple in that when you have a DD pot set up and multiple scheduled payments going into it, it would be nice to be able to add a note/comment so you can see what the purpose of the payment is.

Tied in with that, it would be helpful to allow us to change the amount of these scheduled payments to pots rather than forcing us to delete and add a new one.