Add enterprise plans

config/runtime.exs

lib/plausible/auth/user.ex

lib/plausible/billing/enterprise_plan.ex

lib/plausible/billing/enterprise_plan_admin.ex

lib/plausible/billing/plans.ex

lib/plausible/mailer.ex

lib/plausible_web/controllers/billing_controller.ex

lib/plausible_web/email.ex

lib/plausible_web/router.ex

...ble_web/templates/billing/change_enterprise_plan.html.eex

...plates/billing/change_enterprise_plan_contact_us.html.eex

...usible_web/templates/billing/change_plan_preview.html.eex

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

...le_web/templates/billing/upgrade_enterprise_plan.html.eex

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

...usible_web/templates/email/enterprise_over_limit.html.eex

lib/workers/check_usage.ex

priv/repo/migrations/20211020093238_add_enterprise_plans.exs

test/plausible_web/controllers/billing_controller_test.exs

test/support/factory.ex