Show number of files and size of collection

This addresses two needs for us:

* total files in a collection
* size of the collection in bytes, megabytes, etc.

The first one is easy, but the second required some additional model methods.
ah, ok.  I was trying
```
self.members.map { |gf| gf.file_size.first.to_i }.inject { |sum, n| sum + n } || 0
```
which wasn't working