Add ability to filter out archived repositories (#370)
Two changes in this PR:
1. Store the archived status of mirrored repositories in zoekt-mirror-{gerrit,gitlab,github}
- the archive status is stored in the `zoekt.archived` field of the git config of the cloned repository
- this approach is in line with how the existing `zoekt-sourcegraph-indexserver` represents archived repositories
- the other zoekt-mirror binaries are for upstreams that don't have the notion of an archived repository, so I have not changed them
2. Add a new query option, `archived:` which accepts `yes`, `no`. `archived:yes` will mean only results from archived repositories are returned. `archived:no` will mean results from archived repositories are excluded.
- this is achieved by constructing a query using the `RawConfig` query type