Handle full date and time for AMO's last_login

[jwhitlock]

CTMS expects a date for AMO's last_login, and raises an error when passed a date with time. The fix is one or more of:

1. Convert datetimes to dates in Basket
2. Get AMO to convert to a date
3. Allow a datetime in CTMS and convert to a date
4. Change CTMS to a datetime

CTMS rejected a PATCH update with data similar to this:

{
  "amo": {
    "display_name": "Name with Kanji and emoji",
    "last_login": "2021-06-15T15:20:22Z",
    "location": "A sentence.",
    "user_id": 123
  }
}

The error:

[
    {
        "msg": "unhashable type: 'dict'", 
        "loc": ["body", "amo"], 
        "type": "type_error"
    }, 
    {
        "msg": "invalid date format", 
        "loc": ["body", "amo", "last_login"], 
        "type": "value_error.date"
    }
]