Great deal of refactoring

This PR looks huge but this is mostly due to a lot of *renaming*.

- [x] 6f2ce93ab2612299ee23020eb3d522130d2beeb0 fixes an issue where an *empty* upload would be created when `FastImage` throws an exception. Also improved `uploads` specs.
- [x] 3397c2235f5107f5c73eb17f7ea0e3852544524b displays the size of an attachment in the preview instead of adding it during post-processing
- [x] 36e6345eee7b1f7f3a4103d3f8c724b87425b370 changes all the references to "image uploads" to a more generic "upload" (eg. the icon in the toolbar, renamed a few controller/template/views, renamed some classes and strings...)