Extend iframe support for other entity types: taxonomy_term, media.

Problem/Motivation

Right now, the iframe is limited to just nodes. We need to support other standard core entity types such as taxonomy_term & media that can have stand-alone urls.

Proposed resolution

Modify settings form to support additional entity types and bundles as checkboxes.
Replace node specific variables in the functions with generic entity to support other types.

Remaining tasks

Patch forthcoming.

User interface changes

Settings form modification.