BaseUserService now allows app implementation to use the app's User type

[jskupsik]

Currently, BaseUserService does not allow the implementation to use it's own User:

class AppUser extends HoistUser { ... 

class UserService extends BaseUserService {
    // This does not build, because the return type of `List<AppUser> ` is incompatible with `List<HoistUser>`
    @Override
    List<AppUser> list(boolean activeOnly) { ... 

With this fix, the app's UserService is allowed to type its own methods.

[jskupsik]

Toolbox branch, which shows an implementation that now works due to this fix - xh/toolbox#720