Billing page links: settings "Usage and billing" should land on /billing when the plan is the question #245

mail-pro-admin now has a Billing page (/billing: Free/Pro/Max, Stripe Checkout, the portal). The module's settings page offers one link, dashboard_url() = /instances, labelled "Usage and billing". Usage is on /instances; the plan is on /billing. Either split the link in two or point "billing" at /billing and rename the usage one. Also: the entitlement's plan and daily_send_limit now change when a workspace buys Pro or Max, but the module does not enforce the limit yet (#128); the docs say so.