Add org repo groups to mimic search contexts

Created by: ryanslade

This is a transition issue that will temporarily use repo groups to implement org level search contexts.

Similar to this code: https://github.com/sourcegraph/sourcegraph/blob/edd44b4580cffcd8dc91075162810d978ea1e24d/cmd/frontend/graphqlbackend/search.go#L408-L429

We should also return groups for each organisation the current user is a member of. Orgs should only be returned if they have their own external service and at least 1 repo has been added.

Later, we will replace repo groups with search contexts which will be stored in the database and will be user editable.

Depends on: https://github.com/sourcegraph/sourcegraph/issues/15324