R1:c0aac8267dda

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…