We were using the same logic in many different places, so we're
simplifying the code. I'm not convinced about the method names, though,
so we might change them in the future.
Note using this method for the default tenant in the `TenantDiskService`
class resulted in a `//` in the path, which is probably harmless but
very ugly and it also generates a different key than the one we got
until now. I've added an extra test to make sure that isn't the case.
While this is not strictly necessary, it can help moving the data of one
tenant to a different server or removing it.
Note we're using subfolders inside the `tenants` subfolder. If we simply
used subfolders with the schema names, if the schema names were, for
instance, language codes like `es`, `en`, `it`, ... they would conflict
with the default subfolders used by Active Storage.
