Fixes#1727.
With the server down, apply migrations 0245 and 0246. 0246 will remove
the pub_date column, so it's essential that the previous migrations
ran correctly to copy data before running this.
The code generating pub_dates for messages would fail to distribute them
across days if tot_messages was too large.
We refactor this code as a separate function (for clarity and to unit
test for the bug we're fixing), and change the structure and naming to a
form that more clearly describes what's happening. We also shift away
from the approach of all the float-to-int conversions as this is in
general tricky and bug prone - django's timedelta() handles floats as
arguments, so we take advantage of that.
Mateusz Mandera