Update Recurring Contribution: Indicate direction of amount change in activity subject

When an "in progress" recurring contribution amount is changed (i.e. from $50/mo to $100/mo), an "Update Recurring Contribution" activity is created. Calculate the change and adjust the subject line as follows:

If no amount change:
"Recurring Contribution Updated"

If amount increased:
"Recurring Contribution Updated - increased installment amount"

If amount decreased:
"Recurring Contribution Updated - decreased installment amount"

(see CRM_Contribute_Form_UpdateSubscription::postProcess)