R1:0efce646c95f

Improve tokenizer loading behaviors

Summary:
Ref T4420. Fixes T3309. Two major UX issues here:

- When the user extends a query ("alin" -> "alinc"), we currently hide all the results, then show them again when the new results arrive. This makes the typeahead feel a bit flickery. Instead, show matching results, then add more results when everything arrives.
- When loading more results from ondemand sources, we currently do not give you any indication that things are loading. Instead:
- Show a…