Add ordering options in the list of attachments and order by default using "createdAt DESC"

[mconf-daileon]

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Author Name: Leonardo Daronco (Leonardo Daronco) Original Redmine Issue: 1916, http://dev.mconf.org/redmine/issues/1916

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Add options to order the attachments in a space by: name, author, upload date. The way it's implemented could be similar to how it's done in spaces/index, that involves making a request and letting the controller order the results, or it could be dynamic, using javascript only and ordering in the browser.

Also, the default order should be "createdAt DESC" instead of "createdAt ASC" as it is today.