15610: Fixes layout issues with the collection panel file listing.

15610: Fixes layout issues with the collection panel file listing.

* VirtualTree now just occupies all available space.
* VirtualTree's root container expands so that it occupies the remaining
  height of the browser window.
* Reduced vertical padding on many places to show more information on screen.
* Moved file listing 'hamburger' button next to the 'upload button' to save
  vertical space.

Arvados-DCO-1.1-Signed-off-by: Lucas Di Pentima <lucas@di-pentima.com.ar>