[development] Is a block used anywhere?

[Walt Daniels]

We have hundreds of blocks, some of which were generated for some
particular event and are no longer needed, but there is no way I can find
to tell if a block is in use. Most of our blocks are listed as disabled,
i.e. not in a region, but actually on a panel page or truely unused. If
would be nice if the blocks admin page distinguished these two cases.