R1:f46686ff586d

Implement a Git LFS link table and basic batch API

Summary:
Ref T7789. This implements:

- A new table to store the `<objectHash, filePHID>` relationship between Git LFS files and Phabricator file objects.
- A basic response to `batch` commands, which return actions for a list of files.

Test Plan:
Ran `git lfs push origin master`, got a little further than previously:

```
epriestley@orbital ~/dev/scratch/poemslocal $ git lfs push origin master
Git LFS: (2 of 1 files) 174.24 KB / 87.12 KB
Git LFS…