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

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*NOTE: We have a signed CLA for all of Simon Fraser University (SFU) on file.*