Refactor all common code into base module and leave the sub-module dead simple

Problem/Motivation

In working on #2871646: hook_entity_save support for import and delete API operations it got me thinking that it would be a nice fit to move to a model of the base module having everything necessary to programatically support the RSS and API approach, and then having super light sub-modules for views and api. This is very easy since we're now using the Serialization system, and does not depend on views for RSS approach like we did in D7. The _views sub-module would just be the filter and default view config. The api would just be a hook_entity_save, hook_entity_delete, and alter hook to insert business logic. Leaving it super easy for developers to just turn on the base module and go with their own solution if they want (eg. importing via drush, using a queue, using rules module, vbo, completely custom hook_entity_save, etc.), or turn on a default super simple and lightweight out of the box integration for either RSS or API.

Proposed resolution

Move all non-view related code from _views sub-module into the base module. Move all code from the _api module into the base module. _api module becomes use of the API approach on hook_entity_save and hook_entity_delete.

Remaining tasks

Implement

User interface changes

API settings form will live in the base module and will be available by default. Should add text to explain it's purpose better if it doesn't do that already.

API changes

None.

Data model changes

None.