Adding Entity Framework details for the new tables

[JackLewis-digirati]

Resolves #618

This pull request adds migrations for 2 new tables

• DeliveryChannelPolicies - used to store default and customer specific policies for delivery channels
• ImageDeliveryChannels - stores details of delivery channels attached to assets

[donaldgray]

Given that this will always be a JSON object do we want to convert it on the way in + out to make it easier to use without deserializing it each time?

e.g.

public class DeliveryChannelPolicy
{
  public IPolicyData PolicyData { get; set; }
}

// concrete classes
public class AVPolicyData : IPolicyData {
  public string[] KnownPolicyNames { get; set; }
}

public class ThumbnailPolicyData : IPolicyData {
  public Sizes[] Sizes { get; set; }
}

public class ImagePolicyData : IPolicyData {}

Not sure what those classes would look like so might be something that can be added later.