Endpoint+controller for a remarkup image proxy

Summary:
Ref T4190. Currently only have the endpoint and controller working. I added caching so subsequent attempts to proxy the same image should result in the same redirect URL. Still need to:

• Write a remarkup rule that uses the endpoint

Test Plan: Hit /file/imageproxy/?uri=http://i.imgur.com/nTvVrYN.jpg and are served the picture

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley, yelirekim

Maniphest Tasks: T4190

Differential Revision: https://secure.phabricator.com/D16581