R1:097bcb397061

Make `bin/diviner generate --repository <repository>` accept identifiers

Summary: Ref T4245. This currently accepts only callsigns; prepare it for the bright new callsign-optional world.

Test Plan:
- Ran `./bin/diviner generate --repository 1 --book src/docs/book/flavor.book --clean`, got a good result.
- Ran `./bin/diviner generate --repository 239238 --book src/docs/book/flavor.book --clean`, got an appropraite error about a bad repository identifier.

Reviewers: chad, avivey

Reviewed By: avivey…