Improve checks for recipientMessages

[mpuckett159]

Copied from PR #72

We were getting a lot of log messages like this:

cli_1  | [2021-09-10 04:22:07,410 - signal_scanner_bot.messages - 111][ERROR] Malformed message: {'envelope': {'source': '<redacted>', 'sourceDevice': 3, 'timestamp': 1631247723757, 'receiptMessage': {'when': 1631247723757, 'isDelivery': True, 'isRead': False, 'timestamps': [1631247719921]}}}
cli_1  | [2021-09-10 04:22:07,423 - signal_scanner_bot.messages - 111][ERROR] Malformed message: {'envelope': {'source': '<redacted>', 'sourceDevice': 1, 'timestamp': 1631247723704, 'receiptMessage': {'when': 1631247723704, 'isDelivery': True, 'isRead': False, 'timestamps': [1631247719921]}}}
cli_1  | [2021-09-10 04:22:07,438 - signal_scanner_bot.messages - 111][ERROR] Malformed message: {'envelope': {'source': '<redacted>', 'sourceDevice': 1, 'timestamp': 1631247723471, 'receiptMessage': {'when': 1631247723471, 'isDelivery': True, 'isRead': False, 'timestamps': [1631247719921]}}}
cli_1  | [2021-09-10 04:22:07,464 - signal_scanner_bot.messages - 111][ERROR] Malformed message: {'envelope': {'source': '<redacted>', 'sourceDevice': 1, 'timestamp': 1631247721505, 'receiptMessage': {'when': 1631247721505, 'isDelivery': True, 'isRead': False, 'timestamps': [1631247719921]}}}
cli_1  | [2021-09-10 04:22:07,491 - signal_scanner_bot.messages - 111][ERROR] Malformed message: {'envelope': {'source': '<redacted>', 'sourceDevice': 1, 'timestamp': 1631247726286, 'receiptMessage': {'when': 1631247726286, 'isDelivery': True, 'isRead': False, 'timestamps': [1631247719921]}}}

These are just delivery receipts which we're now seeing due to changes in the Signal API. This PR changes the check to pass these onto the rest of the filters and NOT consider them malformed.

We should have more comprehensive tests for this.