Enqueue file thumbnail UI updates

Fixes CNVS-22819

Test Plan:
* Create a course
* Upload 100+ files into the root files directory
* Navigate to Files in the course and watch the scroll bar as
  files load in
* The page should still be responsive while the files are being
  loaded in, even with 100+ files there

Change-Id: I8b3f9cb1b5622cf38820f8b0916806fd1f5b153b
Reviewed-on: https://gerrit.instructure.com/64789
Tested-by: Jenkins
Reviewed-by: Clay Diffrient <cdiffrient@instructure.com>
QA-Review: Ryan Allen <rallen@instructure.com>
Product-Review: Dan Minkevitch <dan@instructure.com>

Author: Dan Minkevitch

app/coffeescripts/react_files/components/FilesystemObjectThumbnail.coffee

@@ -47,6 +47,8 @@ define [
 
       @props.model?.fetch
         success: (model, response, options) =>
-          @setState(thumbnail_url: response.thumbnail_url) if response?.thumbnail_url
+          setTimeout =>
+            @setState(thumbnail_url: response.thumbnail_url) if response?.thumbnail_url
+          , 0
         error: () ->
           clearTimeout(timeout)

spec/coffeescripts/react_files/components/FilesystemObjectThumbnailSpec.coffee

@@ -89,5 +89,6 @@ define [
       React.unmountComponentAtNode(@thumbnail.getDOMNode().parentNode)
 
   test "fetches thumbnail_url and puts it into state", ->
-    equal @thumbnail.state.thumbnail_url, "sweet_thumbnail_url", "fetches and set thumbnail url into state"
+    @clock.tick(1000)
+    ok @thumbnail.state.thumbnail_url is "sweet_thumbnail_url", "fetches and set thumbnail url into state"