R1:8af5bb117d0b

Basic image thumbnailing

Summary:
This is still very rough but provides basic support for generating image
thumbnails. I need to separate stuff out a bit but I'm going to integrate into
Maniphest before I hit the profile stuff so this seems like a reasonable
starting point.

Test Plan:
Generated some image thumbnails in various sizes.

Reviewed By: aran
Reviewers: jungejason, tuomaspelkonen, aran
CC: aran
Differential Revision: 333