Ref T4100. Share all edge logic code across applications.
- Internalizes the "check that the viewer can see projects" check into edge logic.
- Adds some convenience functions. Some of these aren't really all that convenient, but it's rare that we actually apply project constraints to queries in the applications -- and most of these callsites will go away in the long term -- so I didn't go too crazy with providing a simpler withProjectPHIDs() universal API or anything.