Support uploading directories.

(Code for #19.)
Please test this. It seems to work fine on all the usecases I could think of (this late in the night :smile:).

Needs the equivalent French translations, I hope someone can help me with those.

It's my first time writing coffee, so there are probably places where the code could be more idiomatic, please let me know about those.


Main logic:
* user adds a folder to the input
* figure out the directory structure that needs to be created
* create it (the directory structure)
* upload all the files as usual (just like in the multiple case)

There are progress bars in the modal for all the files that are uploading, and if there's a problem along the way the process will stop with the corresponding modal.