RFC 201: Add UI for configuration

Created by: efritz

The auto-indexer currently supports:

• automatically inferred configuration via project structure
• a configuration file found in sourcegraph.yml in the repository root

In addition to these, we want to be able to configure auto-indexing from the UI so that we can easily configure open source repositories on Sourcegraph Cloud without requiring the maintainer to merge a configuration file. This was a stretch goal that fell out of the master vision of RFC 201.

We need to add room in the database to store configurations for repositories, GraphQL endpoints to query and mutate these configurations, and a UI component in the repository settings page to view and update the configuration.

This will only be available to site-admins in the first release.