Extend civictheme_get_field_value to support File and Media

@richardgaunt for your consideration. Let's discuss priority.

We handle media with `civictheme_media_get_variables` and `civictheme_media_image_get_url` and `civictheme_media_image_get_url` due to its complexity and varied needs for a value.

Perhaps we should add support for media with returning `civictheme_media_get_variables` when passed to `civictheme_get_value`. Thoughts @toby?

Yeh that make sense.

For example, running: civictheme_get_field_value($node, 'field_thumbnail') would return the equivalent of civictheme_media_get_variables('field_thumbnail') after it's done all the checks and balances.

civictheme_media_get_variables returns everything you need to display an image via image_style, or to provide a link to a PDF.