Due to STI nature it is required. Otherwise undesired behavior is possible
in dev and test environments(where usually lazy loading is enabled).
The udesired behavior can be like:
Fetching not loaded yet STI model through its parent model
(e.g. `User.find(service_account_id)` raises `ActiveRecord::NotFound`, because
`ServiceAccount` has not been referenced yet.
`SubclassRegistry` has been removed, because:
1. `.register_subclasses` and `.registered_subclasses` produce unexpected results.
```ruby
# e.g.: Principal -> User -> ServiceAccount
Principal.register_subclass(User)
# Then
Principal.registered_subclasses == [User] # true
User.registered_subclasses == [User] # true
ServiceAccount.registered_subclasses == [User] # true
# Having User as a subclass of User and ServiceAccount seems to be weird.
```
2. There seems to be no big win in have the additional list of subclasses that have to be manually filled.
3. Used in commit apprach seems to be simpler.
It is just calling STI classes explicitly in to_prepare block of configuration.
Pavel Balashou