Clean up times and timezones

[rebeccacremona]

Use django's timezone.now() for simpler timezone handling.

Also, I think there are some places where I'm not exactly handling times correctly: make sure I'm not dropping times anywhere, make sure I'm getting times from subscription creation and not from "now", and check timezone handling one more time for consistency, especially when datestimes are transferred between Perma Payments and Perma.

Good to fix, but not a deal breaker for going live.