Starter tier: Upgrade page remodelling

[RobertJoonas]

Feature flag

This PR introduces a new upgrade page under a starter_tier feature flag. To make the separation cleaner, all existing upgrade page logic will continue to live in modules/functions with a legacy prefix (all of which can safely be deleted once the feature flag is live):

• choose_plan.ex -> legacy_choose_plan.ex
• plan_box.ex -> legacy_plan_box.ex
• plan_benefits.ex -> legacy_plan_benefits.ex
• choose_plan_test.exs -> legacy_choose_plan_test.exs
• Plausible.Billing.Quota.suggest_tier -> Plausible.Billing.Quota.legacy_suggest_tier

Changes

Adds Starter plans on the upgrade page along with some other changes:

• Removes header (Plausible Logo and account dropdown) - replaces it with a simple "<- back to settings" link
• Yearly price is now selected by default (unless the user is already on a monthly plan)
• Yearly plans now also display the monthly price and monthly discount.
• moves the "current cycle / last 30 days usage" and the "what happens if I go over my pageview limit?" paragraph into a FAQ style accordion menu

Tests

• Automated tests have been added

Changelog

• This PR does not make a user-facing change

Documentation

• This change does not need a documentation update

Dark mode

• The UI has been tested both in dark and light mode

[github-actions]

Preview environment👷🏼‍♀️🏗️
PR-5394