Use CSS Grid on photo.html, resolve some Chrome/FF differences.
Use CSS Grid on photo.html, resolve some Chrome/FF differences.

- In Firefox, the image under flex would be full-res height
  instead of staying screen height.
  In this new Grid-based layout the image is the correct size.
  Left toolbox still uses flex, no problems with it.
- Redid the classing of the photo_viewer and eliminated
  photo_img_holder so that all media types follow the same markup.
- Added a CSS variable for tracking narrow mode instead of relying
  on coincidental properties like flex settings.

voussoir/etiquette,voussoir/etiquette,voussoir/etiquette