Consider having images in the Guide open in a modal

[jhung]

Is your feature request related to a problem?

Currently, when an image is selected (e.g. Virtuous Cycles diagram, or Inclusive Design Mapping figures), the image is rendered by loading the image URL in the browser. To go back to the content, one must use the browser's back button.

Originally filed as FLOE-517

Describe the solution you'd like

In some situations, it may be better that the image load in a modal which is displayed in context of the content, and the user selects "Close" or selects outside the modal to dismiss it.

Additional context or notes

This modal implementation will have consequences for accessibility (see Accessible Modal Dialogs). Also these modals may be confusing on mobile devices where the modal takes up nearly 100% of the space (in these situations does navigating back close the image, or does the user have to select "Close?).