R1:d403700e1fc6

Convert all tokenizers to take token/scalar inputs

Summary: Ref T7689. Ref T4100. This advances the goals of removing `loadViewerHandles()` (only 67 callsites remain!) and letting tokenizers some day take token functions like `viewer()` and `members(differential)`.

Test Plan:
- Sent a new message; used "To".
- I simplified the cancel URI construction slightly because it's moot in all normal cases.
- Edited a thread; used "Add Participants".
- Searched rooms; used "Participants".
- Searched countdowns…