Support thumbnailing non-image files and straighten out setup for 'gd'

Summary:
Make 'gd' an explicit optional dependency, test for it in setup, and make the
software behave correctly if it is not available.

When generating file thumnails, provide reasonable defaults and behavior for
non-image files.

Test Plan:
Uploaded text files, pdf files, etc., and got real thumbnails instead of a
broken image.

Simulated setup and gd failures and walked through setup process and image
fallback for thumbnails.

Reviewed By: aran
Reviewers: toulouse, jungejason, tuomaspelkonen, aran
CC: aran, epriestley
Differential Revision: 446