Add a total file size summary to the summary statistics line for the AnVIL Data Explorer.

[bvizzier-ucsc]

We have a request to expose the total size of the files selected to the UI for the AnVIL Data Explorer.

The proposed solution is to add that value to the summary information available in the upper right of the AnVIL Data Explorer's page.

Open to other ideas on where/how to expose this data.

This would also be useful for HCA, and, possibly, LungMAP. (The LungMAP qualification is that we don't have access to the BDC hosted files in LungMAP to obtain their size.)

[achave11-ucsc]

Blocker landed.