Support filtering repos by owner in API

Created by: ryanslade

We need to add the ability to filter repositories by a single namespace here:

https://github.com/sourcegraph/sourcegraph/blob/48625df9c1af26ea3ae9dfa0d9d899bd2bf13df9/cmd/frontend/graphqlbackend/schema.graphql#L2096

Initially we'll only support users but after https://github.com/sourcegraph/sourcegraph/issues/15324 is implemented should support orgs too.

Part of: https://github.com/sourcegraph/sourcegraph/issues/15401