Refactor simplifying frequent template variables

* Use {% with %} to cache frequently-referenced variables in item list
  view templates
* Also change templates/files/[draft|ready]/file_list_item.html
  references to file.url back to file.file.url
  (closes #1585)
Merge pull request #1681 from Stanford-Online/jrbl-1585-file-file-url-master

Refactor simplifying frequent template variables