DLTP-189 Working on UI experience when curation concern encounter generic file without content

UI changes related to empty content on generic file

Adding missing file icon
changing edit and show page of generic file when content is empty

adding spec for empty content

DLTP-189 Wordsmith as part review with Chris

Change button name on edit page when empty file found
Change Thumbnail icon on the generic file with empty content
Disable download and Rollback button in case empty content found

Fixing failed spec

Changes from code review
Merge pull request #39 from ndlib/DLTP-189

DLTP-189 Working on UI experience on generic file without content