workflows: task result templates

Adds templated task results for use in Holding Pen.

Before:
![Image]
(https://www.dropbox.com/s/0sryglpxpesfnh9/Selection_002.png?dl=1)
After:
![Image]
(https://www.dropbox.com/s/pfgo4fl8kltgv78/Selection_001.png?dl=1)

Also adds better documentation and other fixes.
Do we need keys `success_per` and `failed_per`? What about calculating them on client side?