`user_home` option interferes badly with the SSH authentication with public key

[fflorent]

Related to #899 and to YunoHost/issues#2839

Setting the user_home permission can be a big pitfall, as this option enforces the write access for the group to the home directory.

Several thoughts:

• Is this option really necessary or should we drop it? How useful is it?
• Assuming it is useful and we keep it, probably it will be a better place to add this option in the config panel instead of in the manifest's installation questions, because it is too dangerous (and its use discouraged IMHO)
• It should be disallowed to activate this option when the SSH authentication through password is disabled