R1:43bd76336c6a

Use Javelin placeholders and new sorting rules broadly; consolidate tokenizer construction code

Summary:
- We have three nearly-identical blocks of Tokenizer construction code; consolidate them into Prefab.
- Add placeholder support.
- Augment server-side stuff to specify placeholder text.

Test Plan: Verified behavior of Differential edit tokenizers, Differential comment tokenizers, Maniphest edit tokenizers, Maniphest comment tokenizers, Maniphest filter tokenizers, Differential filter tokenizers…