Support additional token entities for email letter task

We've finished getting the email task to use the token processor for existing tokens (contact, contribution) and with a bit more cleanup it is 'easy' to add other entities.

However, there is a gotcha that I discovered on the contribution task.

If I have 2 contributions in the search and you select to send me emails for both I will get 1 email - with details from only 1 of the contributions. This seems wrong. OTOH if the letter had NO contribution specific tokens it would make sense.....