Valid users checks

Exposes POST on /users/ to validate device phone numbers against HaywireDB

Request payload
```
{
    users: [<phoneNumber>, <phoneNumber>]
}
```

Response payload
```
{
    "valid": [<phoneNumber>],
    "invalid":
        {
            "nonMembers": [<phoneNumber>],
            "invalidData": [<invalidPhoneNumberData>]
       }
}
```