Scope: Add subscriptions to give core

[jonwaldstein]

Resolves #6166

Description

This scope is a collection of PRs outlines below that introduces subscriptions as a core concept.

• New: Migrate legacy give-recurring subscriptions into core New: Migrate legacy give-recurring subscriptions into core #6167

Affects

• New LegacySubscriptions domain with give-recurring classes migrated

Testing Instructions

• Make sure you're recurring branch is aligned with this PR https://github.com/impress-org/give-recurring/pull/1111
• Create a donation form with recurring
• Donate with recurring
• Uninstall recurring and donate with the same form
• Now repeat the test with give-recurring's branch not aligned with this one (develop)

Pre-review Checklist

• Acceptance criteria satisfied and marked in related issue
• Relevant @unreleased tags included in DocBlocks
• Includes unit and/or end-to-end tests
• Self Review of code and UX completed

[JasonTheAdams]

@jonwaldstein Please resolve the merge conflicts so we can merge this.

[jonwaldstein]

@JasonTheAdams merge conflicts fixed, this scope is ready.