Store report start/end in UTC

[lboeman]

See: SolarArbiter/solarforecastarbiter-dashboard#462

We currently define start end as something like The end of the analysis period as an ISO 8601 datetime. Unlocalized times are assumed to be UTC. We should convert any offset items into UTC for storage, since we're storing the timezone separately users can convert back as necessary.

Issues arise in report computation when a reports start/end span a DST transition and the reported start and end have different fixed Offsets.