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.

• 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 validation typeahead, got reasonable behaviors?

The error behavior if you delete the repository for a path is a little sketchy still, but roughly okay.