* replace unlisted plans with legacy plans

...and add a legacy plan that has an existing subscriber in Paddle. All
legacy plans are considered generation 1 - meaning that when a user on
one of these plans and they'll go to the upgrade page, then the listed
plans will be v1 for Growth and v3 for Business.

* remove redundant plans_sandbox function

* remove the unused 'scope' argument from Plans.find

* remove unused plan

* add basic test coverage for legacy plans

* add another plan with an existing active subscriber (fix another bug)