R1:76577df50614

Extract textual object list parsing from Differential

Summary:
Ref T2222. Currently, Differential has a fairly hairy piece of logic to parse object lists, like `Reviewers: alincoln, htaft`. Extract, generalize, and cover this.

- Some of the logic can be simplified with modern ObjectQuery stuff.
- Make `@username` the formal monogram for users.
- Make `list@domain.com` the formal monogram for mailing lists.
- Add test coverage.

Test Plan:
- Ran unit tests.
- Called `differential.