R1:2a3c3b2b98fe

Provide `bin/nuance import` and ngram indexes for sources

Summary:
Ref T10537. More infrastructure:

- Put a `bin/nuance` in place with `bin/nuance import`. This has no useful behavior yet.
- Allow sources to be searched by substring. This supports `bin/nuance import --source whatever` so you don't have to dig up PHIDs.

Test Plan:
- Applied migrations.
- Ran `bin/nuance import --source ...` (no meaningful effect, but works fine).
- Searched for sources by substring in the UI.

Reviewers: chad…