Group reports by target account (#8674)

app/helpers/admin/account_moderation_notes_helper.rb

app/helpers/home_helper.rb

app/javascript/styles/mastodon/admin.scss

app/views/admin/reports/_account.html.haml

app/views/admin/reports/_report.html.haml

app/views/admin/reports/index.html.haml

app/views/settings/exports/show.html.haml

config/locales/ar.yml

config/locales/ast.yml

config/locales/ca.yml

config/locales/co.yml

config/locales/cs.yml

config/locales/da.yml

config/locales/de.yml

config/locales/el.yml

config/locales/en.yml

config/locales/eo.yml

config/locales/es.yml

config/locales/eu.yml

config/locales/fa.yml