More aggressively preload on merge request and issue index pages

app/controllers/concerns/issuable_collections.rb

app/controllers/concerns/issues_action.rb

app/controllers/concerns/merge_requests_action.rb

app/controllers/projects/merge_requests_controller.rb

app/helpers/issuables_helper.rb

app/views/projects/merge_requests/_merge_request.html.haml

changelogs/unreleased/optimize-mr-index.yml