Localize billing screens

lib/plausible/billing/billing.ex

lib/plausible/billing/subscription.ex

lib/plausible_web/controllers/billing_controller.ex

lib/plausible_web/email.ex

lib/plausible_web/templates/auth/user_settings.html.eex

lib/plausible_web/templates/billing/upgrade_to_plan.html.eex

lib/plausible_web/templates/email/over_limit.html.eex

...lausible_web/templates/email/trial_upgrade_email.html.eex

..._web/templates/email/yearly_renewal_notification.html.eex

...igrations/20210513091653_add_currency_to_subscription.exs

test/plausible/billing/billing_test.exs

test/support/factory.ex

test/workers/check_usage_test.exs

test/workers/send_trial_notifications_test.exs