[Security] Blind write access could cause problems

@Jermolene @pmario

I've come across what I think may be a corner case, or may be something much more substantial. I'm not sure yet.

Consider the scenario where a teacher has a bag for each of his students which they can write to and he can read all their bags loaded in one recipe (maybe he enforces that they all name their tiddlers with some prefix, somehow, which is fairly easy to do with some minor UI tweaks).

Now each student has write access to one of the bags in the recipe, but cannot read most of them. However, a student may be able to come up with an attack which somehow allows them to modify their own bag in such a way that when the teacher loads his recipe, or clicks a button, or who knows what, it dumps all the tiddlers of all the students into that one student's bag.

Obviously the partitioned bags feature I keep talking about would enforce username prefixes at the server level to avoid trusting the user, and we can prevent bags from loading modules, so most of this can be mitigated quite well, but a few vectors may still be possible.

Question: Is there any sequence of widgets which would be able to cause a wiki change simply by transcluding a tiddler?

Start by reproducing the issue's question: test whether transcluding a tiddler or using a sequence of widgets can cause a wiki change in the MultiWikiServer scenario. Trace whether a student-controlled bag can write or expose tiddlers from other bags; done when the attack is confirmed or ruled out and the affected access boundary is documented.