Bug 1041873: Add mass deletion tool.

I don't think it will be a major maintenance burden. Any new dependency should already be cascade deleted correctly, and any deleted relations will have to be deleted elsewhere as well.

In any case, the code is not just there for cascade deletion. We need to display a summary of these related objects in the confirmation page, so we get this for more or less these two lines.

And, yes, we do currently allow users to delete accounts. Unfortunately, that currently often fails. As do our attempts to delete many accounts. All of which should be fixed (which I think @cvan already filed a bug for).

But I think that, given the fact that the related object query code is necessary regardless, and the lack of this tool is an ongoing problem for both my team, and for the ops team, which is having to manually run queries to delete these objects, this is an acceptable short term solution.