Support view_mode requests

Problem/Motivation

We could support a view_mode argument in the page request to further tune what exact content is output.
For example:
https://example.com/node/1.md?view_mode=entity-only: Markdown Version (Only the node Entity Render without the header, footer, and other surrounding elements).
https://example.com/node/1.md?view_mode=full: Full Markdown Version Including the header, footer, and other surrounding elements.

Proposed resolution

Add support for view_mode arguments.

Remaining tasks

User interface changes

none

API changes

Data model changes