Fix inability to upload files without Flash

When Flash is unavailable, the fallback Add File link is not clickable,
because the invisible div#swfupload_holder is blocking it. Hide to fix.

Test plan:
* Use a browser with no Flash installed (or Flash disabled)
* Go to the Files page in any course
* Click on the Add File link
* Confirm that the basic HTML file uploader appears below
Merge pull request #366 from sfu/fix-noflash_upload

Fix inability to upload files without Flash