Querying AlertManagementAlerts with the assignee field
through the GraphQL API would result in a N+1 query.

This fixes the query by using BatchLoader::Graphql to
group the assignee queries at the end of multiplex query,
only once all alerts have been processed. The accomodate
authorizations, this logic circumvents our GraphQL auth
layer, asserting the same authorization only after
the BatchLoader has completed its work.