Remove `repos.list` from site configuration

Created by: tsenart

Background

In transitioning to configuring repositories to be synced using only scoped external services (part of #914), the top-level site configuration option repos.list needs to be removed.

Compatibility

This change-set is not backwards compatible and it doesn't automatically migrate top-level repos configured with repos.list to be under any already configured external service.

Admins will have to manually re-configure repos that were individually added to repos.list to the appropriate external service configuration via the correspondent UI section. How-to do this will be documented and this breaking change reflected in the CHANGELOG.

We could auto-migrate repos.list to their respective external services, but this wouldn't succeed for all supported code hosts, since not all of them have already implemented functionality to select individual repos (e.g. github.repos).

Because of this, I am uncertain if it's worth the effort to do auto-migration for those code hosts that have these feature but leave out others. @nicksnyder and @sqs: Please chime in on this question.

EDIT: As per https://github.com/sourcegraph/sourcegraph/pull/1391#issuecomment-447040370, documentation and CHANGELOG is indeed the way to go.

Deployment

• sourcegraph.com: No special considerations.
• Dog food: No special considerations.
• Customers: Re-configuration of repos.list needed using external services.

Notes

I'm adding some question comments to the code, one per commit, that I'll seek answers for before finishing this PR. Those commits with titles beginning with delete! will be left out of the final change set.