Storage deduplication for file uploads

As it currently stands, every attachment uploaded on a Fluxer server is stored as-is, even if said attachments are the same thing (for example, when I send that silly cat gif from my saved media ten times in a row). Storing multiple copies of the same file wastes valuable storage space, which is not great for the mainline Fluxer and worse for self-hosted instances.

Make it so that Fluxer can detect when a file is uploaded multiple times, and link all occurrences of the said file to a single copy. Perhaps this can be done by comparing hashes - Fluxer should already be generating them to make sure the file is not banned from the admin panel.