Store UTC dates in the DB

[openbullet]

Issue #1038 was resolved by converting date filters to local time, but this shouldn't be a long-term solution, since the database can be ported over through different timezones and, since we're not saving any timezone info, the date would not be valid anymore.

This needs to be resolved at the HitStorageService level in OpenBullet2.Core, saving the date as UTC. We need to consider the implications of this in other parts of the codebase (e.g., in the web and native client hits table display).