R1:376c85a828c8

Make subproject/milestone watch rules work better

Summary:
Ref T10349. These got sort of half-weirded-up before I separated subscriptions and watching fully. New rules are:

- You can watch whatever you want.
- Watching a parent watches everything inside it.
- If you're watching "Stonework" and go to "Stonework > Masonry", you'll see a "Watching Ancestor" hint to let you know you're already watching a parent or ancestor.

Test Plan:
- Watched and unwatched "Stonework".
- Watched and unwatched…