Fix timezone comparison issue

[irhkang]

When ENABLE_SAVE_LAST_DECRYPTION_TIME is enabled and the eye icon is clicked, confidant returns a 5xx because the last_rotation_date field of the response is trying to compare a tz unaware timestamp with a tz aware timestamp. Essentially what was happening was:

# Assume someone just rotated a credential
c = Credential.get(123)
c.last_rotation_date = datetime.now() # no tzinfo
c.save()   # Pynamo will add tzinfo=UTC to last_rotation_date
...
# One day later
...
c = Credential.get(123)
c.last_decrypted_date = datetime.now() # no tzinfo
c.save()  # Pynamo will add tzinfo=UTC to last_decrypted_date
c.next_rotation_date # uses the unsaved object (`c`) and compares `last_decrypted_date` (no tzinfo with last_rotation_date, which has the tzinfo because pynamo added it

Essentially, had we refetched the object (did another Credential.get(123)), we wouldn't have this problem but we were using the unsaved object.

Instead of using datetime.now() we create a new method that automatically adds a tzinfo.

(Side note: Why python's datetime.utcnow() doesn't have a tzinfo in it is beyond me)