Galaxy Reports: sort by history space is broken

In Histories > Histories and Datasets per User, if I try to sort by history space it leads to an Internal Server Error.
The log displays:
`TypeError: 'int' object has no attribute '__getitem__'`

We can however find this information via: Users > User disk usage

Reproduce the failure in Histories > Histories and Datasets per User by sorting on history space, then inspect the reported TypeError in the server log. The fix is complete when this sort no longer produces an Internal Server Error and history-space results are displayed correctly.

Written by the indexing model from the issue text.