Restore Subscriptions Functionality and Front-End and Backend

This is a complete list of current subscription extension issues sorted by blocking issues (does not allow the effective usage of the extension) and non blocking.

Solidus Version:
Current

To Reproduce
Install solidus with starter front-end.

Blockers

• #304 Breadcrumbs broken on starter front-end after installing extension
• #303 #243 Subscriptions are loaded for not subscribable products
• #302 Can't add subscriptions to cart: Validation failed, Subscibrable can't be blank
• PR #300 #294 Depreciation Warnings
• #269 #243 Add default quantity 1 to starter front-end subscription implementation
• #253 Subscriptions do not work with variants
• #311

API Completition

• #224 Index and Show on Subscription API
• #297 Add API Description

Non Blocking Issues

• #297 Add API Description
• #278 Subscriptions in cart should increase in quantity if two identical subscriptions are added
• #270 Skip subscriptions intervall
• #267 Cannot update payment methods/sources attached to a subscription
• #232 Configure Reminders
• #128 Subscriptions should work without addresses

To be discussed

• #209 Remove Churnbuster: Should be tested

Start by reviewing the linked blocker, API, non-blocking, and discussion issues, especially #269, #224, #297, #278, #270, #267, #232, #128, and #209. Confirm which unchecked items remain applicable and whether their requirements are decided. This issue is complete only when the listed subscription front-end, backend, and API gaps are resolved or otherwise closed.