Refactor IngestTypeConfig

[afred]

• Use factory for IngestTypeConfig - use skip_create and initialize_with to create a non-ActiveRecord factory to DRY up ingest_type_config_spec.rb
• Use ActiveModel validations to validate which config options are required and what types they should be.
• Remove checks on the reader class and ingester class in IngestTypeConfig. If the classes aren't defined by the time they are used, default error of uninitialized constant should be good enough.
• Could probably also get rid of several custom errors dealing with invalid config if we are using ActiveModel::Validations
• Can still run validate! at the end of IngestTypeConfig constructor to immediately validate.

The ingest config file is part of the interface for interacting with batch ingest, therefore we should provide validation and meaningful feedback when a config file is invalid. Currently, the validation is ad hoc, but could be made a lot cleaner with ActiveModel::Validations.