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.
I think we concluded in IRC that a simpler, more efficient impl of this would be:

```ruby
members.reduce(0) { |sum, gf| sum + gf.file_size.first.to_i }
```