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 "Stonework > Iteration IV".
• While watching "Stonework", visited "Iteration IV" and saw "Watching Ancestor" hint.
• Created a task tagged "Stonework > Iteration IV". Got notified about it because I watch "Stonework".

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10349

Differential Revision: https://secure.phabricator.com/D15280