Add size to the /bags API responses
Add size to the /bags API responses

This is for two reasons:

1. So users sending an update can see the size of files already in the
   storage service, allowing them to send a correct Payload-Oxum.

2. So users downloading a bag can work out the complete size of the bag
   before starting a download, and check they have enough disk space.

wellcometrust/platform-api,wellcometrust/platform-api,wellcometrust/platform-api,wellcometrust/platform-api