Fix trial expiration

Separating plan_level from community to a new table for the sake of saving some data about when communities have changed their plan levels.
Hmh... This is also a bit problematic and not according to our newest architectural plans.

As we talked in the cottage trip, the ideal would be to have "loosely coupled" tables. Plans are definitely one of the clearest feature, which we all agree that it should be in different service. And now the ActiveRecord relation makes it hard to split it to its own service.

How should we solve the issue? The best thing would be to remove the relation and write a separate "service" layer, but that requires additional work...