BUG: Push#created_at being displayed as UTC time.

[afred]

The bug

1. The date on the AMS production server defaults to UTC, and we don't change it when building stacks.
2. That date is used for ActiveRecord timestamp fields automatically, e.g. Push models.
3. The result is that UTC date/times are being stored in the DB.
4. We are not adjusting the date time for display, so displayed times are UTC (but there's no indicator).

To reproduce

1. Navigate to a Push (/pushes/[id]) where you already know about when it was created.
2. Note that the displayed date/time is UTC, and not a local time.

Background

1. I think storing the automatic ActiveRecord timestamps in UTC is a good idea, as it's agnostic of timezone.
2. Converting to the correct timezone should be the priority.
3. Defaulting to EST/EDT is fine for now, but...
4. Ideally we should first try to get the users actual timezone, then default to EST/EDT. If that's not hard, go for it in this ticket. Otherwise, just add the default EST/EDT conversion.

Done when

Date/time for Pushes should at default to EST (or EDT if we are in daylight time).