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 rest of the PhabricatorFileURI callsites and get rid of
them, but I think this is the most substantive one.

Test Plan: Profiled Maniphest task list, queries went from >100 to a handful.
Explosion of multiderp. :/ Looked at some views with profile photos to verify
they still render accurately.

Reviewers: jungejason, nh, tuomaspelkonen, aran

Reviewed By: aran

CC: aran

Differential Revision: 921