Enhanced submission system

- Using "Ladda" buttons, with progress bar embedded in them (see them
live here : http://lab.hakim.se/ladda)
- The submission API now works like this :
1. Uploads into a temp directory
2. Checks if a file with the same name exists
3. If it doesn't, puts the uploaded file in the /packs directory

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NOTE : I don't know why GitHub keeps saying that the "loader.gif" and "loader2.gif" were changed, as I did nothing on these files. Moreover, GitHub for Windows doesn't want to sync if I replace the one I have on my computer with the *original* ones, because it detects no changes.