Implement a new transaction log & dashboard views

This PR includes the following changes:

* Refactoring of the transaction logging to improve the details available during the payment process if things go wrong. This means replacing the old logging models with a ``Transaction`` corresponding to an eWay transaction. Additionally, each request (on the backend) is logged in a ``RequestLog`` corresponding to a specific transaction.

* Adding a set of views that can be hooked into the Oscar dashboard showing a list of ``Transaction``s as well as details for each individual transaction.

* Extending the new ``Transaction`` to include a status corresponding to the processing status of the transaction. This allows for tracking dangling transactions that might occur in some edge cases where the eWay Javascript confirms a payment but never returns back to us, so we don't know about it.

* Aadding a new management command that allows checking for dangling transactions and flagging them as "suspicious". These transactions are highlighted in the dashboard view and can be handled accordingly.

@a-musing-moose Would you mind checking this PR? If you are happy with it can you please squash it before merging, the history is slightly messy. Thanks!