Use modal windows to show progress bars for uploads

[mconf-daileon]

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Author Name: Leonardo Daronco (Leonardo Daronco) Original Redmine Issue: 1490, http://dev.mconf.org/redmine/issues/1490

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Currently, when a user uploads a logo, a progress bar is shown below the upload button with the name of the file and the progress of the upload. See the image attached.

It would be better if this progress bar was shown in something like a modal window, on top of everything, in the middle of the page, and that closes automatically when the upload is done. It would prevent multiple progress bars from being shown at the same time (like in the image attached) and would be more similar to how file transfer work in OSs and other websites.