Refactor current plan data format to make changing it easier

assets/css/app.css

lib/plausible/billing/plans.ex

lib/plausible_web/email.ex

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

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

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

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

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

lib/plausible_web/views/auth_view.ex

lib/workers/check_usage.ex

lib/workers/notify_annual_renewal.ex

priv/plans_v1.json

test/plausible/billing/plans_test.exs

test/plausible_web/controllers/auth_controller_test.exs

test/workers/check_usage_test.exs

test/workers/notify_annual_renewal_test.exs