Review Board 4.0 regressed working with site directories containing

custom commands. There were two problems:


The usage of exec() no longer worked, since we introduced a nested

   function within the same function calling that. Turns out this isn't

   legal in Python.


Our method of injecting custom commands no longer worked.


To fix this, we now have a new self-contained method responsible for

grabbing the custom command. We then run it by using newer support for

providing custom command classes to run (which does require that we

create a specialized subclass of ManagementUtility).
With this, we should have a more future-proof way of loading custom

commands.

Created a site directory with custom commands and reproduced the failure

condition. Tested that the new code worked on Python 2.7 and 3.x.
Verified that standard commands continued to work, and that both standard

and custom commands also continued to show the correct help output.