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