Add query to determine available bulk operations

Created by: eseliger

Currently we do a best-effort in the frontend, but it would probably be simpler and more resilient to do that from the backend.

availableBulkOperations(batchChange: ID!, changesets: [ID!]!): [BulkOperationType!]!

With such a GraphQL query we could change the dropdown in the frontend to not show an action when changeset(s) are selected but only once the dropdown button is hit. It would then query the available operation types and display those.

The components added in https://github.com/sourcegraph/sourcegraph/pull/22658 could maybe be reused here.