R1:03fb1887d301

Fix file URI perf regression

Summary:
The CSRF changes meant that we can't generate a file URI with just its PHID
anymore, and converted a mathematical function into a service call.
Unfortunately, this caused massive perf problems in some parts of the
application, critically handles, where loading N users became N single gets.
Derp derp derp. Remedy this by doing a single multiget. This substantially
improves performance of many interfaces, particularly the Maniphest task list.

I need to go through the…