Remove formatter plugin; use third party settings instead.

Problem/Motivation

The responsive image formatter plugin is internal, meaning it may change freely between minor core releases. We're extending an internal plugin here, which is not good.

Proposed resolution

Instead of extending the formatter plugin class, we can simply add a third party setting to the existing formatter plugin.

Remaining tasks

Move existing logic to third party settings.

User interface changes

Just show a checkbox on the existing responsive image formatter.

Data model changes

There will no longer be a custom formatter plugin! Any sites that might be using this module will have to stop using the plugin prior to upgrading!