add default_repos table and use it if it is nonempty and the repo: field is missing

Created by: ijt

This PR makes progress on https://github.com/sourcegraph/sourcegraph/issues/4992#issuecomment-517559931. The goal is to make queries without a repo: field work on sourcegraph.com.

Before:

After:

Test plan: Unit test for defaultRepos.List. Manual testing:

1. In psql add a row to default_repos for some repo in the repo table.
2. Run a query with something to be found in that repo, but without a repo: field.
3. The search results should only show matches for that repo and no others.
4. delete from default_repos; and check that search results appear for other repos as well.