Added possibility to disallow create to a user

I would like to add feature requested by issue #2696.

This is the approach I chose:
I add a "disallow" option in quotas.
This option removes writes on user's "files" folder, so it display a message to the user saying he can't write in this directory.
In my commit d5042cbc59e33dbb88aa174f17ecd884d1008e6a, I transform quota from 0 to unlimited to avoid displaying top message saying user hasn't enough free space in his directory. (He's considered as a limited user without write permission, so it's worth telling him he hasn't enough space, 'cause he can't write).

I don't know how to display a correctly translated "disallow" in quota list. Please help me.

P.S: It's my first commit to this project, please correct me if I don't respect standards/norms. I'm wide opened to all remarks.
If I misspoke, don't hesitate to tell me :-)

P.S2: I'm french, sorry for my english.