R1:2b4c551b0ef9

Provide a global router for Ajax requests

Summary:
Fixes T430. Fixes T4834. Obsoletes D7641. Currently, we do some things less-well than we could:

- We just let the browser queue and prioritize requests, so if you load a revision with 50 changes and then click "Award Token", the action blocks until the changes load in most/all browsers. It would be better to prioritize this action and queue it immediately.
- Similarly, changes tend to load in order, even if the user has clicked to a specific file.