R1:a0262c0b4f14

Remove `tokenizer.ondemand`, and always load on demand

Summary:
Ref T4420. Tokenizers currently operate in "preload" or "ondemand" modes. In the former mode, which is default, they'll try to load the entire result list when a page loads.

The theory here was that this would slightly improve the experience for small installs, and once they got big enough they could switch to "ondemand". In practice, several issues have arisen:

- We generally don't have a good mechanism for telling installs that they…