Clean up issuable lists

app/assets/stylesheets/pages/issuable.scss

app/assets/stylesheets/pages/labels.scss

app/views/projects/issues/_issue.html.haml

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

app/views/shared/_issuable_meta_data.html.haml

changelogs/unreleased/issueable-list-cleanup.yml

features/steps/project/merge_requests.rb

spec/features/issues/filtered_search/filter_issues_spec.rb