Improve Diffusion behavior for externals

Summary:

• Feature request from Airtime that I missed in the feedback notes, came up yesterday.
• Identify git submodules as "FILE_SUBMODULE", not "FILE_NORMAL".
• Link git submodules to an external resolver endpoint, which tries to find commits in tracked repositories.
• Identify git symlinks as "FILE_SYMLINK", not "FILE_NORMAL".
• Add folder, file, symlink and externals icons.

Test Plan:

• externals/javelin is now identified as a submoudule and links to Javelin, not identified as a file and links to error.
• bin/phd is now identified as a symlink.
• Interfaces have pretty icons.

Reviewers: btrahan, cpiro, ddfisher, keebuhm, allenjohnashton

Reviewed By: btrahan

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1975