Add display of file details to the 'manage file' form.

Sometimes, as site administrator, I need to check the detail of files that have been uploaded to the site and are listed in the 'Manage files' listing (which is a very useful addition to BD). This is particularly the case for two of my sites where a large number of image files have been accumulated, several being added each month.

Difficulties arise because two or more images may be listed with the same name because they have been used in different places in the site. They may be in different folders or Backdrop may have assigned them a revised basename. In some cases they may be differently sized versions of the same image.

I would like the 'Manage file' page that is linked from the 'Manage files' listing to display the available details for the file, in particular the uri which shows the location and basename, and also show the size of the file.

This can be done with a small addition to files\file.pages.inc. Alternatively a view might be used. I have prepared a PR backdrop/backdrop/pull/2762 which adds to file.pages.inc. This would benefit from improved styling.