refs https://github.com/TryGhost/Team/issues/579
The previous `members_allow_free_signup` -> `members_signup_access` migration made a direct correlation between the toggle `true/false` to `all/invite` under the assumption that behaviour between the two settings would be identical. The assumption was incorrect and the behaviour is changing so `invite` forces invite-only mode, stopping all front-end signup to free or paid plans with the free plan now being disabled via the portal plans setting.
- check existing `members_signup_access` setting and if it's `'invite'` migrate it to `'all'` where signup should still be possible. The "invite-only" mode should only be active if certain conditions are met:
- Stripe is not configured ("allow free member signup" off and no Stripe showed "invite-only" in portal)
- Stripe is configured but no plans are selected in portal (no plans showed "invite-only" in portal)
- when migrating `'invite'` to `'all'`, also remove `'free'` plan from the `portal_plans` setting to avoid previously paid-only sites unexpectedly showing a free plan on signup
refs https://github.com/TryGhost/Team/issues/634
- find earliest backup file created when a 4.3 migration was run, if found use the `members_allow_free_signup` value from there to change `members_signup_access` from `'all'` to `'invite'` if necessary