The datagrids code was old and crufty, and made some bad typing

assumptions all throughout. A few of these have bit us in the past, when

search indexing bots threw garbage at our datagrids and caused HTTP 500

errors. The mess of types have made it hard to reliably work on this

code, and since it needs some upcoming work, it was time to fix that up.
This change adds typing and documentation all throughout the datagrids

code. In the process, bad typing assumptions and scope errors were

fixed.
There's a lot of old, bad design patterns in this code, but this doesn't

try to change any of those. It's just building enough of a foundation to

start iterating on.

Djblets and Review Board unit tests pass.
Tested all of the datagrids used in Review Board.