Make "source" field output more configurable

Merged DaveD requested to merge DaveD/cdntaxreceipts:source-field-2 into master

Sometimes the source field contains "Entered by staff", or something that doesn't make sense on a receipt. Or sometimes you don't want source at all. You can do this with hook_alter_receipt, but it seems common enough to provide a UI setting. This PR allows entering a token which could be used to include, say, a custom field or the financial type instead of source, or to blank it out.

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