get_user_root_node_folder picks the oldest public folder
Previous to this commit, get_user_root_node_folder
would fail with
a UserHasTooManyRoots
in case this was called on a tree which had
more than one "public" folder.
We fix this by taking the oldest available node, which will always correspond to the notion of "root", being the first ever created.
This fixes a problem that @anoe was having after merging !350 (merged).