R1:d2e5afb0959c

Use application PHIDs in Releeph, plus more

Summary:
Ref T2715. Ref T3551. Ref T603. This does a few things, but they're all sort of small:

- We commonly use a `getX()` / `attachX()` pattern, but have very similar code in the `getX()` method every time. Provide a convenience method to make this pattern easier to write.
- We use `willFilterPage()` in many queries, but it currently is called with zero or more results. This means we have a lot of "if no results, return nothing" boilerplate. Make it call…