This change adds a new module for converting legacy per-repository bug

tracker state into the new configuration model. It classifies each

repository (in-repo, standalone tracker service, or custom URL),

extracts unprefixed settings, and creates ConfiguredBugTracker

configurations scoped to the repositories that produced them.
Identical standalone and custom configurations are deduplicated by

fingerprint. These fingerprints never span Local Sites. Each created

configuration records its fingerprint in extra_data, making the creation

idempotent. In-repo configurations aren't deduplicated, because they're

bound to individual repositories anyway.
A repository's default bug tracker is only assigned when unset or when

it was itself derived from legacy settings, so manual edits are never

overwritten. The legacy fields are left untouched.
Nothing calls this yet. The save-time sync, management command, and

upgrade step follow in later changes.

Ran unit tests.