content icons from content type plugins' partials

fixes #294 
@bamnet you might want to revisit the submission_tile.  I'm sure this change brought it down a little because it uses the render_partial_if which, I believe, does a filesystem check each time its called.  I wonder if we could cache in that method...?
@brzaik I'm sure the class changes don't comply with your level of expertise, but you should be able to see from it what was intended and tweak/relocate it as necessary. [And I'll be able to see what you change and do better next time.]
You can visually see these in the new content tabs, the my content page, and the submissions grid.  When the weather, simple-rss, remote-video, and iframe get upgraded, you'll see those icons also-- I'm going PR those commits next.