Assign receipt date to online contribution receipt template

Knowing whether a contribution has previously been receipted can affect the text of a contribution notification - especially in the context of sending a receipt from the Contribution task as it's likely that it could be an original receipt, or a re-send.

I'm adding this variable to the template at this point from the function called by IPNs, the api call to send a confirmation receipt and the the task (which is only one place for all 3). The contribution page probably uses this flow too but it's not so important as it relates to new contributions so would always be null.

It feels a bit like 'yet another variable add without a plan' - but since I don't have a plan and this is a quick improvement that doesn't warrant spiralling out I will just add it