Remove payment/subscription system and card/storage limits

Fizzy is now free. Remove the entire Stripe billing system,
subscription management, and card/storage limit enforcement.

Removes from saas/: Plan model, Account::Billing, Account::Subscription,
Account::Limited, Account::OverriddenLimits, Account::BillingWaiver,
all subscription/billing controllers and views, Stripe webhook handler,
card creation/publishing limit enforcement, admin account override UI,
usage report rake task, and all related tests.

Removes from main app: Fizzy.saas? guards for subscription panel,
SaaS card footer override, near-limit notices, and saas.css stylesheet.

Adds migration to drop billing tables from the SaaS database.

Non-billing SaaS features (push notifications, signup, authorization,
telemetry, console1984/audits1984) are preserved.

Gemfile.saas

@@ -5,12 +5,10 @@ git_source(:bc) { |repo| "https://github.com/basecamp/#{repo}" }
 
 gem "activeresource", require: "active_resource"
 gem "actionpack-xml_parser" # needed by queenbee for XML request body parsing
-gem "stripe", "~> 18.0"
 gem "queenbee", bc: "queenbee-plugin"
 gem "fizzy-saas", path: "saas"
 gem "console1984", bc: "console1984"
 gem "audits1984", bc: "audits1984", branch: "flavorjones/coworker-api"
-gem "csv"
 
 # Native push notifications (iOS/Android)
 gem "action_push_native"