Make the item hold metrics for each representation

As of today, item metrics are stored and computed only for the latest representation uploaded by the user. This is a problem, specially because it hides the metrics of older representations from the user and from the curator.
To fix this issue, the following must be done:
- Remove the metrics storage in the builder-server. Keep accepting the metrics to not break the API.
- Remove the metrics from the item in the builder-ui when creating a new item. Make it computable for each representation uploaded.
- Change the way the representations are stored when using the item editor. Check if they can be stored into redux to see if we can use them later to compute their metrics.