Rename metadata -> properties (#390)

assets/js/dashboard/filters.js

assets/js/dashboard/query.js

assets/js/dashboard/stats/conversions/index.js

assets/js/dashboard/stats/conversions/prop-breakdown.js

lib/plausible/stats/clickhouse.ex

lib/plausible_web/controllers/api/external_controller.ex

lib/plausible_web/controllers/api/stats_controller.ex

lib/plausible_web/router.ex

priv/tracker/js/analytics.js

priv/tracker/js/plausible.hash.js

priv/tracker/js/plausible.js

tracker/src/plausible.js