repo-updater: Improved repository handling for Github external services

Created by: tsenart

This change-set is a test-bed for using the new repository syncing code with a single external service kind, namely, Github.

Once we nail this one together, @keegancsmith and I will split the remainder of the code hosts between ourselves.

Part of #2025 (closed)

TODO before merging this PR:

• Add feature flag.
• Add and test migrations to drop "global_dep" and "pkgs" tables. These migrations need to come before the migration that adds a unique (external_service_type, external_service_id, external_id) constraint to the repo table because it only knows how to update foreign key constraints on the "discussion_threads_target_repo" table and it'd fail because of the other existing foreign key constraints on "global_dep_repo_id" and "pkgs_repo_id".
• Make api.ExternalServicesListRequest{Kind -> Kinds} change backwards compatible. (Keep old field Kind for the roll-out)
• Address TODO around repos being renamed that are marked as deleted.
• Test feature flag.
• Ensure that repos are always upserted with external_service_xxx columns fully set in the old code.
• Review the PR

Deployment notes

• Deployment on sourcegraph.com will likely require some hand-holding because the migrations will take relatively long.
• Ensure that Kubernetes timeouts don't kill the migrations mid-way.

Follow-up work after merging this PR:

See https://github.com/sourcegraph/sourcegraph/issues/2025