Add GraphQL query for shared groups

Proposal

Currently we are using a REST API to display the Shared groups tab on the group overview page but it will be easier on the frontend if we use GraphQL. It will also allow us to have access to all the required data so the Vue components can render everything. We should make the :id/groups/shared REST endpoint available as a GraphQL query.

Implementation guide

1. Create a SharedGroupsResolver file
2. Add the following to GroupType

field :shared_groups, Types::GroupType.connection_type,
  null: true,
  description: 'List of shared groups this group was invited to.',
  resolver: Resolvers::Namespaces::SharedGroupsResolver