Add system checks to dashboard in admin UI (#15989)

app/controllers/admin/dashboard_controller.rb

app/javascript/styles/mastodon/forms.scss

app/lib/admin/system_check.rb

app/lib/admin/system_check/base_check.rb

app/lib/admin/system_check/database_schema_check.rb

app/lib/admin/system_check/message.rb

app/lib/admin/system_check/rules_check.rb

app/lib/admin/system_check/sidekiq_process_check.rb

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

config/locales/en.yml