Make shift seeds timezone-aware.

[timparenti]

After upgrading to Rails 5+, db/seeds.rb should probably use something like Time.zone.strptime() instead of DateTime.strptime() in order to create timestamps which are properly timezone-aware. This should fix the recurring bug over the last few years which has caused the initially-seeded shifts to end up scheduled 4 hours earlier than desired, since DateTime.strptime() ends up interpreting the local timestamps defined in the CSVs as UTC instead.

Also, grep the repo for all instances of Eastern and New_York to find other similar cases.

[timparenti]

Leaving more breadcrumbs for later:

Once merged, this would also affect "Carnival creation"-style shift seeding in d091159 as well as task seeding in 4b8a0c5 (both from #294).

The only timestamps seeded into the database in this way are shift start, shift end, and task due.