Add an uploading bar to the process

[Comentarinformal]

I've always wanted to do this. It could be done with some jQuery magic. Definitely will try when I have a moment.

[Comentarinformal]

On it, surprisingly more complicated than I thought... Might even have to rewrite backend.

[Comentarinformal]

Front-end basic structure finished. Needs still a LOT of work.

[Comentarinformal]

Prettified the upload progress. Backend still not touched. I wanna finish front first.

[Comentarinformal]

After fighting a bit with the file uploader API, it now handles server errors (should) and user aborting uploads.

[Comentarinformal]

Skin upload backend is fully ready. Working on thumbnail upload backend...

[Comentarinformal]

welp