Only use UTC times

[stvnrlly]

Currently, the application converts back and forth to Eastern Time in a few places, using utilities provided by notifications-utils. This adds complexity and possible confusion since we'll actually be working over multiple time zones.

Once we're UTC-only, we can more confidently show users their local time over in the UI.

[terrazoon]

After team discussion we are going to convert the backend to UTC only, so front end will also be UTC for now. We will wait for user feedback about what they want (user-based timezones, service-based timezones, configurable, etc)

[terrazoon]

#272

[terrazoon]

.