Rewrite using Asset Manager Framework

As demonstrated by the large number of bugs reported for this plugin, the approach of mimicking an attachment ID does not hold up beyond standard use. A more fundamental fix is needed.

This plugin needs to be rewritten from scratch using the [Asset Manager Framework](https://github.com/humanmade/asset-manager-framework) once it's stable.

Update: AMF-WordPress is now available as an alpha version: https://github.com/humanmade/amf-wordpress/

Review the existing network-media-library plugin and the linked Asset Manager Framework and AMF-WordPress projects before defining the rewrite scope. Confirm the framework's stable integration path, then document the plugin areas that must be replaced and the behavior that must remain supported. Done means the plugin has been rewritten against the framework rather than continuing to mimic attachment IDs.

Written by the indexing model from the issue text.