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

Reviewed By: chad

Maniphest Tasks: T10537

Differential Revision: https://secure.phabricator.com/D15436