File download stats: Display user who downloaded the file

### What

User would like to know the actual list of visitors who downloaded the file on "File download stats".

### Why

Some users would like more detailed stats, including who downloaded a certain file. This may not be possible for public websites because they are visible by anyone, but at least for private websites, we may be able to track and show which of the sites' users downloaded the file?

### How

Show within existing "File download stats" section on Calypso.

Start with the existing "File download stats" section in Calypso and determine how its current data is presented. Define how downloaders should be identified for private websites, what should happen for public websites, and consider the work done only when the requested user list is displayed appropriately.

Written by the indexing model from the issue text.