New workflow for sending thank-you notes for contributions

1. Do a search for contributions with no thank-you date (using the existing contribution search form)

2. Select contributions and pick "Send a thank-you note" or similar from the drop-down menu.

3. User chooses between PDF and email, like they do with the Receipt function.

4. The message editor would pop up, offering choice between available message templates, with tokens available to describe the contribution being thanked (including especially amount and date) in addition to normal message template tokens for contact info.

5. Once letter generation is complete, the thank-you-date field on those contributions would be updated.