move offset_date function to a shared place
fix start calcs when new month has fewer days than previous one

changes the start from the very end of the previous month to the
very beginning of the next month. i think this is more correct, but
@esoergel should confirm
make offset_date type unaware
update dimagi-utils
Merge pull request #4083 from dimagi/opm-cz

fix start calcs when new month has fewer days than previous one