R1:a4cc1373d3a2

Use a tokenizer, not a gigantic poorly-ordered "<select />", to choose repositories in Owners

Summary: Depends on D19190. Fixes T12590. Ref T13099. Replaces the barely-usable, gigantic, poorly ordered "<select />" control with a tokenizer. Attempts to fix various minor issues.

Test Plan:
- Edited paths: include/exclude paths, from different repositories, different actual paths.
- Used "Add New Path" to add rows, got repository selector prepopulated with last value.
- Used "remove".
- Used…