Add "Files" table to Activity summary section

[ggeisler]

Part of #672.

Below is a suggested start for the Files tab. As with all of the Activity summary tables, the main goal is to start with something, and we can refine and adjust based on what data is actually feasible to gather and what ideas we might get for showing more useful things.

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• A: Provider column. Link to that provider's page > Uploaded files tab.
• B: The files, total filesize, unique records, and total records columns are exactly the same as the columns with the same name in the table on the Providers page. (We won't show the Edit and Destroy buttons as in that table, so hopefully even though it is duplicative, it will be a bit more compact.) Right-align these columns.
• C: Last upload column. Date/time of the last upload from that provider. Link goes to the upload show page for that upload.

[marlo-longley]

@corylown related to our discussion of caching tab data every hour - the best way to get the file statistics needed for this ticket is from the latest_statistics call to each organization - but these statistics are only updated once a day. I just want to note that the data in this tab could be out of sync with the other hourly tabs. The Last upload column we can keep up to date live, but the file counts would be coming through daily.

Slightly related to #788