Report on total post-backs for MassPay

After #2417, we want to be more careful to confirm that MassPay posting
back actually worked. This is low-hanging fruit for that.
Merge pull request #2494 from gittip/nposts

Report on total post-backs for MassPay
factor out computing actual take amounts
use variable names in interpolation
support using compute_actual_takes() inside a transaction
cache a user's takes in a new column of the participants table
update receiving amounts of members when changing team takes
don't forget takes in update_receiving()